Reading is best for this. You can also listen to audiobooks, whoever likes what) How to keep up - the daily routine, allocating time for reading or listening to books in a daily or weekly schedule will help. Now, with the advent of e-books, it is more convenient and easier than it used to be. Online education has received great development recently - there are a number of resources with online courses on a variety of topics. There would be a desire - and there are a lot of opportunities in the modern world!
For example, cosmonauts study constantly and during their time in the detachment they master a large amount of knowledge and skills. They learn almost all the time between flights. The time of narrow-profile specialists is gone. The broader a person's horizons, the easier he perceives new information. The better he will be able to orient himself in the new situation.
And be sure to get yourself a hobby - a significant number of hobbies one way or another develops our brain.