To use the PowerUps in the game, select the one you want from the bar located on the game grid.
You can identify each PowerUp with their corresponding icon and the amount you have of each of them.
To use any of them, simply select the one you want. Bear in mind that each of the PowerUps can be used once per turn.
If you select 'Magic Play' and you decide not to use the designated word, just select the 'Recall' option on the lower right corner of the screen. When you do so, you'll see that the tiles go back to their original position, so you can form another word with the initial tiles if you want to.