Drag players using the three bars and drop them on the player you want to swap them with:              

To swap in a player from another bracket, click Edit on the player you want to swap out and choose from the drop-down:                           

The player being swapped in must be an entrant in the phase

You may only move and swap players who have not played a match in the current phase.  

Make sure to click Save in the top right corner

You may also use the seeding list on the right to adjust the seeding of the bracket: