Unprotect

Unprotect[s1, s2, ...] removes the attribute Protected from symbols, allowing modification of their definitions.

Examples

Unprotect to modify a built-in:

Unprotect[Plus]
(* Now Plus can be modified *)

Protect[Plus]
(* Restore protection *)

Unprotect custom symbol:

Unprotect[myFunc]
myFunc[x_] := x^3

Please visit the official Wolfram Language Reference for more details.