With the POA, am I able to change any of their previous beneficiary decisions?

My parents are both in senior assisted living centers with health and dementia issues. I have the power of attorney for health and finances. I am aware that they have accounts (IRAs, annuities, mutual funds) with several investment firms, pensions, and social security. How do I find out what happens to this money after one or both of my parents pass? I know that some pensions will continue or partially continue to the surviving spouse, but how do I know? and what happens if my father has named my mother as a beneficiary to a policy and she predeceases her? What questions should I be asking? With the POA, am I able to change any of their previous beneficiary decisons?