To change the decks of the game table, click on the User Options button (see the red arrow in the following image),
then click on the Online tab (see the red arrow in the following image) and click on the image at the right of the label Choose Deck (see the blue arrow):
Click on the deck you like and press the bottom right Ok button.

If you see a partially transparent card deck, it means you have not bought it yet, but you can buy a new card deck anytime by going to the SHOP, where you can click on the Decks icon (see the red arrows below).