StringInsert

StringInsert["string", "snew", n] inserts "snew" starting at position n.

StringInsert["string", "snew", -n] inserts at position n from the end.

Examples

Insert at position:

StringInsert["HelloWorld", " ", 6]
(* "Hello World" *)

Insert from end:

StringInsert["Hello", "!", -1]
(* "Hello!" *)

Please visit the official Wolfram Language Reference for more details.