You can only amend a contract after signing it. This may be to reflect a change in law, but more often it’s to accommodate the needs of one or both parties. For instance, a common reason for an amendment is simply to change a party's name.
You don’t need to make a Contract Addendum before signing a contract because the original terms aren’t effective yet. In this case, speak with the party responsible for drafting the document and make the changes there. Terms typically become legally binding once all parties sign the document.
Likewise, you cannot amend an expired contract because its terms are no longer effective. You’ll need to make a new contract.