Party A prepared a valid will and wants to revoke it. How can Party A revoke the will?

Generally, if you are mentally competent, you can revoke a prior will by destroying it, obliterating it, burning it, or tearing it up. Unless the act of revocation is properly witnessed and recorded, someone may later contend the will was simply “lost” and not revoked, or that you lacked mental competence at the time you “attempted” to revoke your will, and this could give rise to a “will contest”.