How to clean a roof?

Only reason to clean shingles if there is moss or mold growth on them from surrounding trees. No other reason. Cleaning and chemicals can wear down the protective granules exposing the asphalt layer.

I do this myself, to keep your roof in tip top shape, I get a chisel, and carefully remove all moss by hand, then take a leave blower, to remove all the debris. Then I blow out all the gutters, takes a couple of days, but worth it!! My roof is 20+ years old, and looks brand new!

In some parts of the country moss, mold and black algae are very troublesome. These 3 organisms are probably responsible for most roof staining. Do a search for "moss algae killer" to find products. With many of these products moss and algae just dry up and crumble away. Try to avoid getting the product on plants. It's not the same as an herbicide, but most can damage greenery. Wash off any drips from your plants.

If you do this job yourself, make sure to wear shoes that grip the shingles. On one forum the majority of professional roofers said high-top basketball shoes were a good choice. Always have a helper to hand up supplies, or call EMS if there is an accident. Lift supplies to the roof with a rope; don't try to carry them. If your roof is steep build a slide guard using roof brackets and 2x6 lumber. If your roof is very steep you should hire professionals due to the increased danger of sliding.

If you should decide to go with a professional company here is an article that explains the three basic types of roof cleaning: https://www.angieslist.com/articles/3-roof-cleaning-methods-ugly-stains.htm