Change of address for my elderly parents

Hi. How to I update a change of address for my elderly parents? I don't have a log in or know if they have an online tax account. Both have Alzheimer's.

Hi gaffer61,
Write to us at Pay As You Earn and Self Assessment, HM Revenue and Customs, BX9 1AS,
Confirming your parents' previous and current addresses, as well as their National Insurance numbers.
Please also ask them both to sign this letter.
Thank you. 

Thanks for the reply. As mentioned both my parents have diminished mental capacity my mother is able to sign documents but my father not really. I have Lasting Power of Attorney for both my parents for both financial and health matters. As my father is unable to sign a document what do you suggest?