This question can mean several things:
How to monitor employees only during their work hours
You can configure when employees are tracked from the Monitoring Settings. Check out this article to learn how: How to change the monitoring schedule?
How to set up work schedule for your employees
You can configure schedules/rota for employees or positions. Check out this article to learn how: How to create/change employee schedule?
How to add, remove or adjust employee work hours
You can add/remove hours from an employee's total time. Check out this article to learn how: How to add / remove time (work hours)?