I'd say simply start practicing all kinds of questions on HackerRank. Start with the Easy ones, once you feel comfortable and are able to solve them within like 5-10 minutes each, go on to the medium ones. I found some of the medium questions super-hard to be honest, but that's just my 2 cents. The questions in the HackerRank assessment can be from either of the difficulty levels, so try to practice on all levels! Enough practice will get you pretty confident and you'll do well on the actual test!