Click on Store Management
Click on Store Management (This is the 4th option on the left-hand side menu)

Click on Departments
Click on Departments (4th item on the left-hand side menu)

Select Department by Name
Select the department you would like to edit from the Department List menu

Revise & Save Changes
Revise as needed and click the save changes button.

Also refer to https://support.petrooutlet.com/en/articles/3520029-how-to-create-a-department for what each option is.
Send POS Transmission
Send POS Transmission to complete the update.
How to Send POS Transmission :
http://support.petrooutlet.com/en/articles/3520104-how-to-send-point-of-sale-transmissions