Not sure I understand your question.
For example what do you mean by delete. For example given the
string ABC, if I delete the B I end up with AC, a 2 char string. Or do you want A C, a 3 character string with the B overwritten with a space?
For the first case, you'll need to rewrite the file by dragging the data following the end of the deleted data to where the deleted data used to be and then to truncate the file the new length.
For the second case, just overwrite the area to be deleted with an appropriate character that represents deleted data???