The first step is to click on Manage at the bottom left corner of SalesScreen.
A menu will pop up, where you can proceed to click on Budgets or Quotas under the Company headline:
If your budget is currently ongoing, locate your budget on the list of current budgets and click the corresponding Pencil button:
If your budget is upcoming, locate your budget on the list of upcoming budgets and click on the corresponding Pencil edit button:
Once you are shown the Budget details, click the Delete button: