Stuff a string into another, replacing 0 or more characters at a certain position.
Note: start
position is 1-indexed (you start indexing at 1, not 0).
Syntax:
STUFF ( character_expression , start , length , replaceWith_expression )
Example:
SELECT STUFF('FooBarBaz', 4, 3, 'Hello') --returns 'FooHelloBaz'