Yes, you can.
If one of your clients works to different payment terms to all your other customers, you will need to reflect this in the forecast to ensure it is accurate.
To increase/decrease payment terms for one or a number of customers that vary from the default dates, then first of all go into your profit & loss forecast.
Expand your sales category.
Select the client that requires different payment terms by clicking on them.
You will then land on the below page.
Click on 'Payment Terms' under the client name.
From there, you can increase or decrease the number in the 'Days credit' field by typing over the existing value or by clicking the up and down arrows.
You can also update invoice dates, which bank account the payment goes into or the VAT rate.
Make sure to click the green 'save' button in the top right corner to update in your forecast data.