Test the acid first on a small, inconspicuous portion of your patio to ensure that it won't discolor it. Then follow the manufacturer's directions, using goggles, gloves, and protective clothing.
I would rather try a bleach wash before this strong and aggressive cleaner (often used with power wash). Try clorox wash before this extreme product that can do damage.
If you have pets hanging around your back yard you should not use acid wash because they might accidentally end up swallowing some of that and it's not good for their health.
Most home improvement stores sell acid based solutions for concrete and tiles, I would get one of them and follow the instructions. The one I got recommended a deck brush, but I rented this huge rotating brush for the pressure washer and was done in under an hour and it was beautiful.
