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:

You'll be shown the Budget detail.
To select a timeframe within the calendar:

To unselect a specific day within your budget timeframe (for example weekends, red days,…): 

Once you have edited your budget timeframe, click the Update button:

If you want the budget changes to be reflected in your slides on screen, you should manually reload your screens.