ReplaceAll

expr /. rules or ReplaceAll[expr, rules] applies a rule or list of rules in an attempt to transform each subpart of an expression expr.

Examples

Replace all occurrences:

{a, b, a, c, a} /. a -> x
(* {x, b, x, c, x} *)

Replace with patterns:

{1, 2, 3, 4} /. x_?EvenQ -> x^2
(* {1, 4, 3, 16} *)

Please visit the official Wolfram Language Reference for more details.