If you'd like to change the gas type associated with your account, select the Account Icon in the upper right corner of the app:
Then select Profile from the navigation menu and then Edit next to the currently selected gas type. You can choose from Regular-87, Midgrade -89, Premium, Diesel, or Other.
The type of fuel you purchase will be determined when the transaction is verified. The gas type selected on your profile will not affect the cashback you receive. So if you drive multiple vehicles, you don't have to worry about the settings.