If you have existing user and is no longer you need to access the certain user, than you can delete the existing user in simple few steps :
Please Follow the following commands :
you can delete user itself below command.
# deluser username
If you are logged in as another user or non root user than you can follow the following command :
$ sudo deluser username
In case, If you want to remove the users home directory, you can use the following command :
# deluser --remove-home username
If you are logged in as another user or non root user than you can follow the following command to remove the home directory for user :
$ sudo deluser --remove-home username
If you have previously configured the user as sudo users than use following command to remove deleted users form sudo groups.
# visudo
If you are logged in as another user or non root user than use following command :
$ sudo visudo
root ALL=(ALL:ALL) ALL
username ALL=(ALL:ALL) ALL # DELETE THIS LINE