Don't underestimate how much your knowledge and skills can fade when unused, even after a couple of years, let alone twenty. I've experienced this personally when trying to study advanced topics for which I had the prerequisites on paper from many years ago, but which were no longer fresh in my memory.
My general advice is first to check the immediate prerequisites for the area you are trying to refresh. If your recall of those is fairly solid, then find an introductory book, lecture notes - maybe your own old lecture notes if you still have them - or video lectures on your target area (in your case, PDEs) and get to work. Be patient with yourself, as it may take some time to get up to speed. As you proceed, you may find yourself recognizing topics and even recalling definitions, examples or methods that you once learned. Take heart and be encouraged!
If you find yourself hazy on the prerequisites, spend some time on those first. How much time is a balancing act. Ideally perhaps, you would study the prerequisites thoroughly, but time may not permit that, or you might simply get bogged down and lose motivation. Monitor how you are going and when you feel somewhat ready, return to your target area. With any luck, you will now be able to make some headway, dipping back into the prerequisites when you need to. (In your case, you might want to revisit some Calculus, Linear Algebra, ODEs and maybe Numerical Analysis.)