Very easy. Every train downtown except for the J, M, and Z (brown line) runs to midtown. Which one you should take depends on where you are in the Financial District and where you're going in Midtown.
Just make sure you get on a train that's going Uptown/Bronx or Uptown/Queens and check the map so you know which one to take.
It takes anywhere from 15 to 30 minutes, depending on the train.
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