Step 1 - To change the mounting system you have selected you'll first need to open the System Design, found at the bottom of the page.
Step 2 - Verify you're on the Array Placement step of the System Design.
Step 3 - Look to the Properties Panel on the right and select the edit symbol to the right of Racking System
Step 4 - Reopen the dropdown menu and reselect the racking. Finish the process by saving.