On observing closely the decoding is done by connecting the first half of the alphabets to the second half ie.,
A - Z
B - Y
C - X and so on.
It's also seen that if the code contains letters from the first half of the alphabets in capital the decoding happens by switching to the corresponding alphabet in the second half with upper case.
And if the code contains the letters from the second half of the alphabets in capital then decoding happens by switching to the corresponding alphabet in the first half with lower case.
In both cases if the letters are small they are just switched to the correponding alphabet on the other side.
So xvYBUz becomes cebYfa