To edit an account group go to 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.
In the window that appears, you can change the name of the account group and select/deselect accounts from the list. When you are done editing the account group, click "Save group".