subst-char-in-region

subst-char-in-region is a built-in function in `C source code'.

(subst-char-in-region START END FROMCHAR TOCHAR &optional NOUNDO)

From START to END, replace FROMCHAR with TOCHAR each time it occurs.
If optional arg NOUNDO is non-nil, don't record this change for undo
and don't mark the buffer as really changed.
Both characters must have the same length of multi-byte form.