Go to configurations > working hours. A modal appears.

To change working hours

To change working hours, click on the time field as shown below and select the preferred time.

To disable a working day 

To disable support on a specific day, uncheck the selected day.

Click on save.

How to set special hours (Holidays and Special working hours)

Scroll down and get to working hours on holidays

Click on select date field. A calendar appears.

Select preferred date. Click on save.