This lever will be marked with a gas pump icon, but you can always check your owner’s manual if you have difficulty finding it. Once you locate the lever, simply pull upwards and then release. You should hear a pop as your fuel door releases.
After you release your fuel door, exit your vehicle and walk over to your fuel door. You should now have access to your gas cap, which you will need to unscrew in order to fill up your rental car’s gas tank. When you’re done, be certain to screw the gas cap back on tightly until the mechanism clicks to let you know that it’s secure.