StringReplacePart

StringReplacePart["string", "snew", {m, n}] replaces the characters at positions m through n in "string" by "snew".

StringReplacePart["string", "snew", {{m1, n1}, {m2, n2}, …}] inserts copies of "snew" at several positions.

StringReplacePart["string", {"snew1", "snew2", …}, {{m1, n1}, {m2, n2}, …}] replaces characters at positions mi through ni in "string" by "snewi".

StringReplacePart["snew", {m, n}] represents an operator form of StringReplacePart that can be applied to an expression.

Examples

Replace characters in a string:

StringReplacePart["abcdefgh", "XYZ", {3, 5}]
(* "abXYZfgh" *)

Replace at multiple positions:

StringReplacePart["abcdefgh", "*", {{1, 2}, {5, 6}}]
(* "*cd*gh" *)

Please visit the official Wolfram Language Reference for more details.