You revoke Power of Attorney by:
preparing a Revocation of Power of Attorney
destroying the POA document, and its copies, that you wish to revoke (as long as you never gave copies to anyone or told your agent)
following any termination procedures mentioned in the POA document
creating a new Power of Attorney that revokes the previous
You can revoke the document at any time (as long as you are legally competent). You should also notify the old agent and any banks, businesses, and other institutions affected by the revocation.