If you want to decrease the quantity of a card in your collection, check out this article.
Remember to use the grouping options to ensure you are removing the desired number of cards.
To remove a card from the staging area, swipe left on the appropriate card. A red bin icon will appear that you can tap to remove the card.
To delete a card that's in your collection, tap on the appropriate card. Tap the Edit button, scroll down and tap the red Remove button.
To remove a card from the staging area (on the Build page), click on the X icon on the right of the appropriate card. You can also remove all the cards in the staging area by clicking the X icon in the header row.
To remove a card that's in your collection, find the appropriate card and click on it to open the details panel. Click Edit and then Remove. You can remove multiple cards by selecting them and clicking View Selection, then Edit, and then Remove.