Within an account group card, you can change the order of the accounts directly in the list of accounts using drag-and-drop.
If you want to add an account to another card, please proceed as follows:
Click on the Pencil-symbol in the upper right corner > then click on ‘Edit’ for the card
Select the account at ‘Included Accounts’
Reorder the accounts for the card by tapping on ‘Sorting’