qgisqgis-3
When I have vector layer with several objects and label them, can I change the text of individual labels? I have tried the labeling tools and the tool change label but the field with the label text is grayed out.
Is it possible to change individual label texts?
In QGIS 2.0, you can create background rectangles, ovals or custom SVG shapes without the need for a plugin. Just go to Layer Properties | Label | Background section.
If you want to code something. This might be a starting point: http://doc.qgis.org/api/qgspallabeling_8cpp_source.html#l04333
Yes, there is! With string concatenation || you can make label with custom strings, or with values from other fields. In Label with field write expression like this:
'A'||your_field
or use expression builder 
example:
Best Answer
GIS software isn't really designed for adding custom text annotations that have no connection to the source data attributes.
If you wanted to have totally custom labels for each feature that should be stored in a separate field in your layer (e.g.
alt_labelas J.R. suggested in the comments).However some of these other options might help accomplish similar outcomes in some situations:
Label text substitutes You can find this under Labels > Text tab in QGIS 3.x up to 3.8, Labels > Formatting in Q 3.10 onwards. You can import and export lists of substitutions as well to use in various projects.
Using expressions in your text label value Instead of just using a field for your label you can use an expression and add custom rules to add/replace text for a specific feature or class of features as you see fit. Such as
Or
and so on.