To revoke a last will without making a new one, all you have to do is tear it up, deface it, burn it, or destroy it. If this is done accidentally, the last will is not revoked.
An old last will cannot be revived once it has been revoked. If you make a new last will (that revokes all earlier last wills) and then decide that you like your old one better, you need to make an entirely new last will that replaces the new one and mimics the old one. Your old last will is invalid.