You can just use the points as you normally would for any other reservation. However, it means they will NOT book your fixed week at the 12-month mark, because the points are not there/already used.
In other words, fixed week (which really are Guaranteed Weeks!) contracts are exactly like other point contracts UNTIL 12 months before your guaranteed week. At that point,
DVC confirms your Guaranteed Week and moves the points into the booking for that week. Once that happens, you can cancel at any point and get the points back for that week (the points spent on the week, not the points it actually is to book the week. This is where it can get confusing, because once the reservation is made, the reservation will show the number of points it would cost to book that week normally, not the number of points your guaranteed week cost you to purchase. When you cancel, you'll want to call MS to make sure you get the correct number of points back. It did not happen automatically/correctly for me last year when I did so on my account).