You'll have to investigate the pointers for this game - there's some great documents here for that.
Basically you will need to figure out the pointer format so you can either
a.) Increase the length of the string it reads (is there any room after 'END' ? or does it run right into other strings?) by modifying the pointer
b.) Use the pointer to point to some empty (usually filled with FF) space with your 'FINE' string.
c.) Switch around pointers with a string that you don't need the full space of - ie, if there's a 4 character string that you -personally- replaced with a 3 character one, you could instead switch the pointers around. For example let's say BOMB and END are right beside each other. If you wanted BOMB to become BOM, you could write it in place of 'END'. And now we have enough letters from where 'BOMB' is to now say 'FINE'.
So now the strings are 'FINE' and 'BOM' - you just flip the pointers around to literally 'point' to the correct one