A will can absolutely be changed or revoked by the person who created the will as long as that person has capacity. The easiest way to change or revoke a will is to simply tear it up.
You can also change a will by adding to it amendments which are called codicils. You can add to or take away from your will thereby revoking prior portions of your will by simply amending it. It can be revoked or amended or completely sorted anytime as long as the person has capacity.