I had a problem using the app on my phone recently. Luckily, I had printed out the paperwork the night before. Go to the aisle with your car "size" and get in and go (remember to do a walk around of the car first). You give your paperwork (or phone) to the exit booth.
If you are in Orlando they have people waiting to help you at the front of the garage. Ask "any chance of a free upgrade today?" and about 50% if the time you will get one.