How do you go about changing the designated POA person amending an existing will?

It is illegal in all 50 states for a POA to change an existing will or to have one drawn up on behalf of the person they are representing.

Are you asking how you would change a POA you granted and a will that you signed or are you asking how to change someone else's POA and will?

Please clarify so that we can properly answer your question.

Cregada, regarding a Power of Attorney, it is up to the original owner of the Power of Attorney, who had assigned someone to be his/her representative, to make any changes to a Power of Attorney.  Some POA's have a primary representative and a secondary representative, or both are equal.  The secondary can take over if the primary is unable to serve.

Same with a Will, no one can change any part of a Will except for the original owner of said Will.

I see by your profile, you are caring for a person who has Alzheimer's/Dementia. Depending on the degree of memory lost, it might be too late for that person to make any changes. But if the person has times of clear memory, then it would be up to an Elder Law Attorney to deem the person capable of understanding a legal document.