ReplacePart

ReplacePart[expr, i -> new] yields an expression in which the ith part is replaced by new.

Examples

Replace at position:

ReplacePart[{a, b, c, d}, 2 -> x]
(* {a, x, c, d} *)

Replace multiple parts:

ReplacePart[{a, b, c, d}, {1 -> x, 3 -> y}]
(* {x, b, y, d} *)

Please visit the official Wolfram Language Reference for more details.