What has to done to legally revoke a power of attorney? Is there a proper legal form that needs to be filed? If so, how does a person obtain this?

It depends on the language of the power of attorney and the laws in your state. Some POAs are expressly limited in time. At any rate, you will need to revoke the POA in writing. You can generally use the same format for the revocation as the format you used for the original POA. In most states, it is also good practice to inform the agent in writing that the POA is revoked, as well as any institutions who have been directed to abide by the POA.

I suggest you speak to an attorney regarding the legal requirements in your state for a valid POA, and hence, for the validity of a revocation, including whether the revocation needs to be notarized, etc.