There are three ways to edit a forecast.
Click on Forecast from the sidebar navigation to view all the forecasts that have been generated for your store. Here you will see the forecast for each variant each month. You can edit the sales quantity directly by updating Forecast Override. If you would like to update it relatively with a percentage instead, you can update Percent Override.
Once you are done with the changes, remember to click on Save Changes to save your changes.
If you would like to increase all the sales quantity by a percentage, you can select the variants (rows) and then edit them in bulk.
Note that this facility will only be available if the previous bulk action has completed running.
Clicking on any row from the Forecasts or Replenishments screen will open up the Forecast Details popover for that variant. You can jump over to the Forecast tab and edit the forecast over here.