I am completely mystified as what to do and totally admire those who have completed this. I have changed a single letter in row "X", column "Y". The new letters alphabetical position is "Z" and
(X*Y*Z) / (X+Y+Z) = 4.125
and the instances of the new letter in the grid = (X+Y+Z)/2
I hope that doesn't give too much away!!!
BUT, I have no idea what the picture that needs to be shown is, nor the term associated with B. I'm also not too sure what the instruction from D actually means!!
C'est la vie.