A power of attorney (POA) is a document that reflects an agreement with someone else who acts on your behalf as your agent. Assuming you are mentally competent, simply the desire to revoke the agreement, and the authority bestowed thereby, is enough to cancel a power of attorney. Practically speaking, however, you need to communicate your revocation in writing and take steps to make sure the old power of attorney can no longer be used.
Destroy copies of POA. Tear up and destroy a power of attorney document to cancel or revoke the authority granted therein. To fully revoke the POA, all existing copies should be destroyed. Getting all copies will likely involve communicating with your agent and some third parties.
Communicate revocation to agent in writing. Tell your agent (the party granted authority in the POA) that you are revoking her authority. This can be done in person or over the phone but should be followed up with a written letter explicitly revoking the authority. You should also request all copies of the POA in her possession so they can be destroyed.
Communicate revocation to third parties. Communicate your revocation of the POA to any third party that had a copy of the document. This might include banks, realtors, stock brokers or other professional organizations. They should be instructed to no longer honor the POA and that all copies should be destroyed.
Execute a new POA (optional). You can revoke an old POA by executing a new one that explicitly revokes any old agreement. This is a common method of revising or amending a POA. Copies of the old document should still be destroyed to avoid confusion.