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.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.