Here is an example of a form to revoke all prior wills.  You shouldn't do this, however, unless you have a new will drafted to replace the one you want to revoke.  Also, don't instate a new will unless you've revoked the old one first because a will written without revoking a prior will makes the new will a codicil to the old will (see the Free Legal Aid Codicil to a Will form for a better understanding).