A Revocable Living Trust can be changed at any time. You can modify it with a trust amendment if you have second thoughts about a provision in the trust's terms, such as if you change your mind about who should be a beneficiary.
An Irrevocable Trust can't be changed by the grantor after the agreement has been signed and the trust has been formed and funded. For the most part, it's forever, although there are a few rare exceptions.