Updating security??

Having trouble finding the right endpoints for updating a user security profile in iMIS cloud (20.3.90.44742). I want to:

change someone from full user to public user
and/or
change an expiration date
mark them as disabled

Have tried /UserSecurity, it doesn't return the expiration date. /User doesn't seem to allow PUT updates (or a valid _execute option). What am I missing?

Thanks much.