# login as root and make sure that there are no active processes for user
usermod -l NEW_USERNAME OLD_USERNAME
# if user is running kill user w the following command
userUID=$(id -u OLD_USERNAME) && pkill -U $userUID
sudo su - username
$ whoami
user1
$ su - user2
Password:
$ whoami
user2
$ exit
logout
sudo su - username
sudo chsh -s /bin/zsh <username>
$ sudo -u <user> <command>