When a durable power of attorney for health care or living will is given to you voluntarily by another person, you always maintain the ability to revoke it so long as you are legally competent. One of the easiest ways to do this is to consult directly with your estate planning lawyer. Your lawyer might recommend the creation of a written statement that formally revokes this document. 
Once you have created this written statement you might need to provide it to everyone who has a copy of your durable health care power of attorney. You should provide a copy of the revocation to the former power of attorney agent and every health care provider that might have previously had dealings with your former power of attorney agent. It is recommended that you destroy all copies of a revoked durable power of attorney for health care. However, you need to follow these additional steps to ensure that all of the aspects of closing out this initial power of attorney have been addressed.
As you can see, it can be somewhat complex to revoke a power of attorney but if it is the right decision for you at this point in time, you may need the support of an estate planning lawyer to guide you through the process and ensure that you have taken care of all of the details. Bear in mind that when you revoke a power of attorney and do not establish a new one, there is no one appointed in this role to make decisions on your behalf if you are unable to do so.