control the backspace key in an existing C++ software

I use a software to memorize lessons. The program hides some letters and the user has to find them. When the backspace key is stroken, the previous letter of the selected word is removed ; instead the removed letter should be the letter given by the user for the previous word. Example : Hell_ the worl_ -> Hella the worl_ (a typed instead of o by mistake) -> backspace -> Hella the wor__ (current result) >>>> Hell_ the worl_ (expected result). Start parameters must be set as on the attached picture; A test file [login to view URL] is attached.