You can delete a budget only if you are its owner.
To delete an individual budget:
Navigate to the main Budgets screen.
Locate the budget of interest.
Select the three dots menu (⋮) at the rightmost end of the budget entry.
To delete multiple budgets at once:
Select the checkbox of each budget that you want to delete.
If you have Owner access to all of the selected budgets, you can use the Delete button next to the Filter to perform a batch delete.
You'll be asked to confirm the deletion before it's executed.