To remove a tag from a card, open the deck containing the card, then go to the Cards screen from the menu, and select the card.
On the Card screen, the card's tags are listed in the Tags section.
On mobile devices, swipe left on a tag to reveal an "X" button. On desktops and laptops, right-click on the tag to reveal the "X" button.
Press the "X" button to remove the tag.