Is it permissible for one of the contracting parties to amend the terms of a contract they concluded two years ago?

The contract is the law of the contracting parties, as long as it does not violate a Shari'ah ruling. Neither party may unilaterally amend the terms of the contract except with the consent of the other party, in accordance with the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their stipulations." Contracts vary in terms of their binding nature: some are binding on both parties, some are permissible for both parties, some are disputed, some are permissible and eventually become binding, and some are binding on one party and permissible for the other.