My daughter has POA and my husband just died. She wants to put me in a rest home, can I change POA? I'm still mentally competent.

Having Power of Attorney does NOT give her the ability to decide where you should live. It only gives her the ability to make financial and contractual decisions on your behalf.
Some POA's do include healthcare provisions but those powers would only be with respect to medically related decisions if you were unable to make those decisions for yourself.
And yes, as long as you have capacity you can revoke the existing POA and have a new one drawn.

POA is only activated if you are deemed not mentally competent. If you have not been deemed as such, then she has no say.

The answer is yes, if you are mentally competent you can change the POA. Let us know a little more. It may be that although you are mentally competent that your daughter doesn't feel that you can physically take care of yourself as needed. If that is the case, talk to her about hiring some in-home help if you want to stay in your home longer.

But think about what she's suggesting, Stillliving. Is she really talking about "a rest home" or is she encouraging you to move to an assisted living facility? Some are very nice---sort of like living in a resort hotel, with all your meals, housecleaning, laundry, etc. taken care of, while you enjoy the companionship of other people, activities, etc. Since you're still up and around, maybe you should visit a few and see what they offer and what they cost. I hope I can afford a nice one when my time comes. My mother is in an inexpensive nonprofit facility---not what I would have preferred for her, because it's rather shabby---but even so she likes having people around, feels safe because she won't be alone if she falls or gets sick, and really, really enjoys having someone else do all the cooking and cleaning .

What's going on with your health that makes her want to do that? There are other choices you know than what people used to say in the old days as a 'rest home'.

Your profile says you are caring for your grandmother. I don't understand, why you would even be considered ill enough for a nursing home..