Hi there, Rebecca.
Once the budget is already saved, QuickBooks doesn't allow us to edit it. I know a workaround for this. I'd suggest creating a new budget by class and use the old budget as the reference. Please refer to the steps which my colleague @Jeno_P provided above.
Also, you can check this article on how to create, edit, and manage budgets.
Don't hesitate to post in the Community again if you have other concerns. We're always here to help you. Have a great day!