Delete a char with a new char

Hello,
I send in Delphi4 a char with sendmessage(memo.handle,WM_CHAR,{char},0)
to a memofield. Know I want to overwrite another char, if they are on
then same place where the Cursor is (like INS-Mode on the keyboard). How
i can simply do it???
Thanx
Frank
f.s...@mvnet.de