The STUFF function inserts a string into another string. It deletes a specified length of characters in the first string at the start position and then inserts the second string into the first string at the start position.
If the start position or the length is negative, a null string is returned. If the starting position is longer than the first string, a null string is returned. If the length to delete is longer than the first string, it is deleted to the first character in the first string. For example:
SELECT STUFF('abc', 2, 3, 'xyz') ---- axyz (1 row(s) affected)