I recommend transcendental meditation. It’s very basic. Pick any nonsensical phrase. I chose “oohm chah kahh.” You close your eyes for twenty mins and repeat that phase in your head. You can repeat parts of it and change up the rhythm by elongating the vowels or whatever. Focus on the phrase as much as you can. Your mind will wander but always come back to it. You do that once or twice a day.

There is a bit of controversy of the science and spiritual side but I find that the way to improve focus is to limit stimuli and practice focusing.

Outside of that, sleep is probably more critical.