How to Change an Employee, who was put through as a Leaver, back as an Active Employee?

When making an employee a leaver, they stay as an active employee until the night of the date of the "leave date" when the system does an overnight batch process. 

To remove the leaver status, go to:

People > Employees

Once there, go to the Actions column of the employee in question, and select 'Change leaver':

Or, you can go to the employee's card and select the same 'Change Leaver':