usermod

Modifies a user account. More information: https://manned.org/usermod.

Change a user's name:

usermod -l {{newname}} {{user}}

Add user to supplementary groups (mind the whitespace):

usermod -a -G {{group1,group2}} {{user}}

Create a new home directory for a user and move their files to it:

usermod -m -d {{path/to/home}} {{user}}