After picking your fantasy game you can trade players in and out of your team;
Click on the '- / +' buttons on the pitch view or team selection box to drop/pick a player;
Click on 'Update Lineup' to confirm (save) your changes;
If you do not press 'Update Lineup' your changes to your team are not saved;
You get 20 free trades per game;
You can make Unlimited free trades before the start of the first game Lockout;
You can only trades players in the Trade Window, i.e. the time period each week when the game is not locked.
Screenshot: My Team