To edit or add a new site/store, click the Parts drop-down (1) then click the Sites/Stores option (2):
If you wish to add a new site, then click the green New Site button (1), if you are wishing to edit an existing site then click the row of the existing site (2):
The next screen will be the save regardless of if you are editing a site or creating a new site.
Enter the site name in Label (1) and the Date Open (2), the rest of the fields are optional. You can also assign a group to the site (3), along with specific vehicles (4). Once you are finished editing then click Save and Close (5):