How do I switch my Moms iCloud on a device to mine

If you know her access information, download anything important (photos, etc.) you can also do this from iCloud later.

Then follow these instructions. https://support.apple.com/en-us/HT201351

If you do not know her access information you'll need to follow the requesting information provided by the other forum member.

Start here => How to request access to a deceased family member's Apple accounts - Apple Support