MapAt

MapAt[f, expr, n] applies f to the element at position n in expr.

MapAt[f, expr, {i, j, ...}] applies f to the part at a nested position.

MapAt[f, expr, {{i1}, {i2}, ...}] applies f at multiple positions.

Examples

Apply a function at a specific position:

MapAt[f, {a, b, c, d}, 2]
(* {a, f[b], c, d} *)

Apply at multiple positions:

MapAt[f, {a, b, c, d}, {{1}, {3}}]
(* {f[a], b, f[c], d} *)

Square a specific element:

MapAt[#^2 &, {1, 2, 3, 4}, 2]
(* {1, 4, 3, 4} *)

Please visit the official Wolfram Language Reference for more details.