Only team captains can remove players from the team's roster.
Click on your team name in the top left corner. Next, under Team Settings, click on Roster
Next, on the Player you'd like to remove, click on the 3 small dots on the right side of their name and select Delete Player:
Note: if the player you are deleting has existing supporters the player will be moved into a "Deleted players" group but will still be shown at the bottom of the player list. They will not however be counted towards splits on the budget app.