Can POA change beneficiary in bank accounts?

[POA Curious]

My uncle designated me as sole beneficiary of one of his bank accounts. His step daughter has POA (don't know what type). We live in California and the accounts are in a CA bank. Q: Can the step daughter change the beneficiary or close the accounts and take over the proceeds? My uncle is in a nursing home and starting to show signs of mild dementia but is still mostly aware and present. 

[MoJo]

It depends on what the POA says, and what the state law of the state in which he was resident when signing says.  A POA is a complex legal document - and should be reviewed by counsel....