How do you re-learn a subject that you've started to forget?
If you took notes when you first learned the material, then start with those, and perhaps summarize them. Then, or if you don't have notes, I'd suggest going back to the exercises and doing them again, starting with the first chapter, rather than a random approach. Perhaps a few from each chapter. If you get stuck, then review the material enough to get to a solution.
If you really did know it, then it should come back with a bit of practice. Hopefully you weren't just kidding yourself on the first &quot;learning&quot;. Then it is much harder.