Can you update/edit a budget from a previous/past financial year?

How are you doing today, stefan-ammann?

Yes, you can modify and update the budget as long as you're the administrator. 

Here are the steps:

1. Go to the Gear icon.
2. Select Budgeting under Tools.
3. Click the drop-down arrow below the Action column.
4. Select Edit.
5. Save or Save and Close.

I'm also including an article where you can check for more details on how to edit and manage budgets.

You can always reply to this thread if you need further help with the Budgeting.