The eigenvalues of an upper triangular matrix are the diagonal entries. Thus, the characteristic equation of $A$ is $\lambda^n$. According to the Cayley-Hamilton theorem, this implies that $A^n = 0$.
For aesthetic considerations suggested by Michael Hoppe, I'll rename $K$ into $n$ and $y_{K+1}$ into $x_{n+1}$, so the matrix whose determinant you're searching is
$$A=\begin{pmatrix}
x_1 & 0 & \dots & 0 & y_1 \\
0 & x_2 & \dots & 0 & y_2 \\
\vdots & \vdots & \ddots & \vdots & \vdots \\
0 & 0 & \dots & x_n & y_n \\
y_1 & y_2 & \dots & y_n & x_{n+1}
\end{pmatrix}.
$$
First method. Use determinant expansion with respect to the last column to get
$$\mathrm{det}(A) = x_1...x_n x_{n+1} + \sum_{i=1}^n(-1)^{n+1+i} \mathrm{det}(A_i)$$
where $A_i$ is the matrix $A$ deprived of its last column and $i$-th row. For example,
$$A_1 = \begin{pmatrix}0& x_2 &0& ... &0 \\
0 & 0 & x_3 & ... & 0 \\
\vdots & & & \ddots& \vdots\\
0 & & ... & & x_n\\
y_1 & &... && y_n \\
\end{pmatrix}. $$
Using row expansion for $A_i$, it is easy to see that $\mathrm{det}(A_i)$ is equal to $(-1)^{n+i} y_i \prod_{j \neq i} x_j$, which yelds
\begin{align*}\mathrm{det}(A) &= \prod_{i=1}^{n+1} x_i - \sum_{i=1}^n y_i^2 \prod_{j \neq i, j\leqslant n}x_j \\
&= \prod_{i=1}^{n+1}x_i \left( 1 - \sum_{i=1}^n \frac{y_i^2}{x_i}\right).
\end{align*}
Edit (second method, hint). This last expression suggests another method (maybe it's not working) : suppose that no $x_i$ is zero. Note $X = \mathrm{diag}(x_1, ..., x_{n+1})$ and $Y = A - X$, so that $A = X+Y = X(\mathrm{Id}+X^{-1}Y)$. Then, $\mathrm{det}(A) = \mathrm{det}(X) \mathrm{det}(\mathrm{Id} - X^{-1}Y)$. Now, all you have to do is to compute $\mathrm{det}(\mathrm{Id} - X^{-1}Y)$. Maybe there's a simple ay of doing this (I don't know).
Best Answer
Using the cofactor expansion along the first column we get $$|A|=(-1)^{1+1}a_{11} \left |\begin{matrix} a_{22}&a_{23} & \cdots & a_{2(n+1)} \\ 0 &a_{33} & \cdots & a_{3(n+1)} \\ \vdots & \vdots & \vdots & \vdots \\ 0 & 0 & 0 & a_{n+1n+1} \end{matrix}\right |$$ (Note that remaining terms in the expansion are zero.)
Using the induction hypothesis (you know det of the $n\times n$ matrix), we get
$|A|$=$a_{11}a_{22}a_{33}\cdots a_{n+1n+1}$