I have not done the body regimen as I do not care about my backne, is what it is. However, I know how all of the products work and I am sure that it will work for your entire body. Use enough Benzoyl Peroxide and Glycolic Acid to cover the areas in which you are applying to.
It should work the same on your back/body as it does on your face. Start off with an amount that you're comfortable with (think small, then amp up to larger amounts). If your skin tolerates it, then you can keep adding a little bit more. I also recommend using a BP cleanser on your back/body.
From my experience, The Regimen addresses a multitude of issues I've had, so I have no doubt it will work on your upper arms. For instance, after I shave my legs, I tend to develop ingrown hairs (gross, I know), but after using Dan's BP and AHA cream on them every night, I noticed vast improvement in the texture of my skin there. I didn't think it would work, but the BP and AHA really helps exfoliate the skin.
Good luck and let us know if you see any improvement! 
On 5/1/2017 at 0:46 PM, joaquinakarol said:I have a bunch of tiny bumps on my forehead that won't go away. Should I buy the regimen.
This is common, i rarely seem to have trouble with acne on my forehead, just genetics i suppose. But if you only have these small bumps on your forehead, you may be able to get away with just treating it with a mild topical, see a dermatologist and see what they say.