By default, the accounts list in the sidebar will list the accounts and account groups in the order they were created. However, you can change this if you wish.
There are three ways of re-ordering accounts and account groups:
swipe left on them to reveal "Reorder" button,
enter account edit mode by clicking the edit button at the bottom of the accounts list,
bring up the contextual menu and select "Reorder".
You will then see handles to the right of each account or account group. Click and hold on them and drag them around to re-order them. There are a couple of things to note here:
dragging an account under an account group will put that account into the group. Similarly, dragging an account out from an account group will remove the account from that group,
account groups cannot be nested - this means that you cannot put an account group within an account group,
accounts that do not belong to any group will always be listed at the top, above account groups. This cannot be changed.