This easy to do, you need to give more information.
The date is on a table?
the Date is only on the Form?
What is the starting date?
The specific amount of days: is it fixed? is it on the form/Table?

A couple of ways to add days to a date:

With Math:
   MyDate = MyDate + 3

With DateAdd:
   MyDate = DateAdd("d", 3, MyDate)

Replace MyDate with the name of your Date field.

This video may spark a nice way to set up your form.
Date Buttons

Let me know what you think.

Thanks Kevin & Sami,

The date is on a table & is displayed on a form.

This date is an expiration date that needs to get updated usually from one Thursday to the next, the thing is that I don't always update the field on Thursday but I always want it to go to that coming thursday