When user leaves the company, you might need to disable them to restrict their access in the future. Also, user might be automatically locked out of the system by inserting password incorrectly too many times - in these cases, you might need to activate the user again. Whatever the case, user statuses can be changed from your System Managament.
To do this:
Go to your "System Management" and to "Users" menu.
Find the user and click on the "edit" button to edit their settings.
Find "Status" field from the "User account" section, change the status to the necessary one.
Scroll down and save.
User status has now been changed!