This translates your page layout into KOMA's terms. It may not make the requested changes because I don't know what they are meant to be. Your example does not include any chapters, let alone chapter titles, and it is impossible to guess which distance you are talking about or what is meant to align with what.
Note that, as originally configured, your layout will be inconsistent because both fancyhdr and scrlayer-scrpage will adjust headheight as required, changing the page dimensions in unpredictable ways. To avoid this, you need to make the height at least 33pt and to tell geometry about it.
Note also that scrlayer-scrpage is designed to cooperate with typearea. By using geometry, you lose the advantages of this. Consider whether you could use KOMA's typearea instead.
Here's the initial translation:
\documentclass[12pt,headsepline,footsepline]{scrreprt}
\renewcommand{\familydefault}{\sfdefault}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{geometry}% do you really need this? Better to use KOMA's setup if possible
\geometry{verbose,tmargin=2cm,bmargin=3.5cm,lmargin=3cm,rmargin=2.5cm,headheight=33pt}% the value 33pt is obtained from console warnings output by scrlayer-scrpage - this is the minimum required value, given the inclusion of the graphic - again, it would be better if you could use typearea for this
\usepackage[demo]{graphicx}
\usepackage{xcolor}
\definecolor{mygreen}{RGB}{23,156,125}
\usepackage{scrlayer-scrpage}
\setkomafont{pageheadfoot}{\upshape}
\setkomafont{pagehead}{\slshape}
\setkomafont{headsepline}{\color{mygreen}}
\setkomafont{footsepline}{\color{mygreen}}
\pagestyle{scrheadings}
\automark{chapter}
\ihead{\leftmark}
\ohead{\includegraphics[height=1cm]{IMG/Logo.JPG}}
\ofoot{\thepage}
\cfoot{}
\chead{}
\begin{document}
Hello
\end{document}
I am guessing that you want the same footer on plain pages as on other pages. This can be achieved using the starred versions of \cfoot and \ofoot, and adding plainfootsepline to the class options:
\documentclass[12pt,headsepline,footsepline,plainfootsepline]{scrreprt}
\renewcommand{\familydefault}{\sfdefault}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{geometry}% do you really need this? Better to use KOMA's setup if possible
\geometry{verbose,tmargin=2cm,bmargin=3.5cm,lmargin=3cm,rmargin=2.5cm,headheight=33pt}% the value 33pt is obtained from console warnings output by scrlayer-scrpage - this is the minimum required value, given the inclusion of the graphic - again, it would be better if you could use typearea for this
\usepackage[demo]{graphicx}
\usepackage{xcolor}
\definecolor{mygreen}{RGB}{23,156,125}
\usepackage{scrlayer-scrpage,kantlipsum}
\setkomafont{pageheadfoot}{\upshape}
\setkomafont{pagehead}{\slshape}
\setkomafont{headsepline}{\color{mygreen}}
\setkomafont{footsepline}{\color{mygreen}}
\pagestyle{scrheadings}
\automark{chapter}
\ihead{\leftmark}
\ohead{\includegraphics[height=1cm]{IMG/Logo.JPG}}
\ofoot*{\thepage}
\cfoot*{}
\chead{}
\begin{document}
Hello
\chapter{Some chapter}
\kant[1-10]
\end{document}
I'm guessing that the further change you want concerns the alignment of the \leftmark in the header. I'm not sure how this constitutes 2 changes, mind, since aligning with the bottom of the image will also reduce the distance....
So, I'm guessing you want something like this:
In which case, it is easiest, I think, to simply define the header in one go, setting the other parts of the header empty:
\documentclass[12pt,headsepline,footsepline,plainfootsepline]{scrreprt}
\renewcommand{\familydefault}{\sfdefault}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{geometry}% do you really need this? Better to use KOMA's setup if possible
\geometry{verbose,tmargin=2cm,bmargin=3.5cm,lmargin=3cm,rmargin=2.5cm,headheight=33pt}% the value 33pt is obtained from console warnings output by scrlayer-scrpage - this is the minimum required value, given the inclusion of the graphic - again, it would be better if you could use typearea for this
\usepackage{graphicx}
\usepackage{xcolor,calc}
\definecolor{mygreen}{RGB}{23,156,125}
\usepackage[markcase=upper]{scrlayer-scrpage}
\usepackage{kantlipsum}
\setkomafont{pageheadfoot}{\upshape}
\setkomafont{pagehead}{\slshape}
\setkomafont{headsepline}{\color{mygreen}}
\setkomafont{footsepline}{\color{mygreen}}
\pagestyle{scrheadings}
\automark{chapter}
\ihead{\leftmark\hfill \includegraphics[height=1cm]{example-image-a}}
\ohead{}
\chead{}
\ofoot*{\thepage}
\cfoot*{}
\chead{}
\begin{document}
Hello
\chapter{Some chapter}
\kant[1-10]
\end{document}
EDIT
In response to the question concerning changing the headers, you can simply redefine \ihead (or \ihead*) as you wish. Do note, however, that doing so is likely to be confusing to readers. The point of running heads is that they contain information which is consistent throughout the document (except on special pages such as the first pages of chapters etc.). Also, the headers should not distract from the body of the document.
\ihead{\includegraphics[height=1cm]{example-image-b}\hfill\includegraphics[height=1cm]{example-image-a}}
\kant[11-14]
\ihead{\leftmark\hfill \includegraphics[height=1cm]{example-image-a}}
\kant[15-20]
EDIT EDIT
You can produce roughly the same page layout using typearea by commenting out the calls to geometry and adding
\areaset[5mm]{155mm}{272mm}
However, if you are stuck with a requirement to use precisely the dimensions given, then you may need to stick to geometry even though it does not cooperate with KOMA. You need to be a bit more careful to make sure you specify adequate headheight etc. in that case.
Best Answer
Unfortunaly there is no MWE in the question.
Using
scrheadingsthe header consits of 3 elements: a left-aligned, a centered and a right-aligned. The centered element overlays the left one and the right element overlays the centered element. Therefore your text (or header) width picture in the centered element overlays the left-aligned\headmark.As a workaround you can use the right-aligned element:
Example:
Note that in one-sided printing, only right-hand pages exist, and LATEX designates these as odd pages regardless of their page number. Therefore
\rohead(right on odd pages) or\ohead(outer on all pages) is enough.You could also define a new layer for the background picture and add this new layer before the other ones.
Example: