You can delete an account group by going go the "Account groups" page under "Accounts". Here, you will see a table with the the account groups that you have created.
To edit one of the groups, click the three dots next to that account group, and click "Edit group" in the menu that appears.
You need to confirm that you want to delete the account group by clicking "Delete account group" in the window that appears. Please note, that you do not delete or remove any accounts by deleting an account group.