;for this example the array's size, the data we want to write, and where we want to write the data, are all known in advance.

ld hl,Array ;hl points to the 0th element of row 0.
ld c,5      ;one-indexed row length
ld b,0      ;set bc = row length
add hl,bc   ;now hl points to the 0th element of row 1.
inc hl      ;now hl points to the 1st element of row 1.
inc hl      ;now hl points to the 2nd element of row 1. This is where we planned on storing our new value.
ld a,20     ;get the value 20 which we want to store here
ld (hl),a   ;store 20 into the desired slot. (Retrieving a value is the same process except we skip the step above and
            ;     execute "ld a,(hl)" at this point instead.)