Click on your avatar (a photo of you or the initials of your name) in the top/right corner.
Click on the ‘Organization’ menu.
Choose ‘Board Groups’ from the left-side menu. You’ll see the list of boards (the default is the ‘Boards’ tab).
Choose the ‘Groups’ tab. Here you’ll see a list of your board groups.
Click the ‘–’ icon next to your board group, then click the ‘Remove Group’ button to confirm.
If there’s a board inside your group, you’ll see this option to reassign your boards. After you click the ‘–’ icon, choose the board group you want to reassign the boards to, then click the ‘Reassign Boards & Remove’ button.