How do we make an amendment to the existing will?

My sister and I recently became co-durable power of attorney for our Mother who was diagnosed with Altzheimers last year. We both want to make some changes to the existing will in regard to my Mother's house. What does this entail and do we need to have this done through my Mother's lawyer or can we write up a "contract" ourselves and have it notarized and still be legal. Thank you.