If you father in law is still lucid and mentally competent he can revoke the previouse poa and make a new one. At the same time he should also give you a healthcare surrogate appointment so that you can make health care decisions when he is unable. So, you need (1) written revocation of existing poa; (2) new durable power of attorney; and (3) healthcare surrogate appointment. These are written documents that are witnessed and notarized. Call an attorney who does wills and estates and the attorney can prepare the documents. Most attorneys do not do house calls, but you might be able to coordinate a visit to the lawyer's office when you are taking your father in law to the doctor's office.
I agree with Mr. Byrd's response. In addtion to his response, however, an attorney is not necessary but is strongly recommended. Many times when people complete documents such as POAs on their own, mistakes are made and often come back to haunt them. Keep in mind there are different POAs. In Florida, there is Durable Power of Attorney which would provide power to you when your father in law is incapacitated (not of sound mind) as well as capacitated. A Springing Power of Attorney would provide power to you only in times when your father in law is incapacitated. In order to execute the correct Power of Attorney, certain and specific language needs to be included.