I've taken the open-book renewal test for years, since I never had the 360 days needed to avoid it.
Not very hard, one book is general captain stuff and seamanship (some of the questions rather dated, like asking about Manila rope, and surprisingly no TVMDC or plotting stuff the last couple of times, 20 questions total. The several questions about sailing (my license is aux.sail) were pretty kludgy, some about schooner rigs you seldom see these days, could stand updating Second book is Rules of the Road, 30 questions. Since it's open-book, and now with Google to help you, I think they're pretty easy. you need 90% on both. Actually helpful to get back into the book every 5 years.
Since your post is a few months old, I guess you know this by now. I just joined here so thought I'd answer anyway. Got my renewed passport-type license about a month ago, so if any questions about renew-by-mail, please ask. I found it wasn't as bad as the old days, and you have a tracking number where you can check status of your renewal on the NMC web site.
Renewals for licenseholders like us, who don't have a big employer or union who does renewal frequently, are really a "do it yourself" experience. The NVIC they have about Medical waivers is very helpful, since the Guard is concentrating much more on this since the Staten Island Ferry and Cosco Busan casualties.