You can make changes to your will or revoke it at any time. There are, however, some very important rules to follow. One way to make changes to a will is to make a codicil, which is an amendment to a will. Another way is to make an entirely new will, which revokes and takes precedence over any older wills. A codicil is a separate document and must be signed and witnessed just like a regular will. Because of these formalities, it is usually easier to make a new will.
Be sure not to make any changes or markings on your will after it has been witnessed and signed. This is absolutely vital. If you cross out a person's name or add writing to a will that has already been signed, you risk invalidating the whole will.