By default, the budget list in the sidebar will list the budgets and budget groups in the order they were created. However, you can change this if you wish.
There are three ways of re-ordering budgets and budget 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 budget or budget group. Click and hold on them and drag them around to re-order them. There are a couple of things to note here:
dragging a budget under a budget group will put that budget into the group. Similarly, dragging a budget out from a budget group will remove the budget from that group,
budget groups cannot be nested - this means that you cannot put a budget group within a budget group,
budgets that do not belong to any group will always be listed at the top, above budget groups. This cannot be changed.