Replace

Replace[expr, rules] applies a rule or list of rules in an attempt to transform the entire expression expr.

Replace[expr, rules, levelspec] applies rules to parts of expr specified by levelspec.

Examples

Replace the whole expression:

Replace[{a, b}, {a, b} -> x]
(* x *)

Replace at specific levels:

Replace[{a, b, c}, x_ -> f[x], {1}]
(* {f[a], f[b], f[c]} *)

Please visit the official Wolfram Language Reference for more details.