Final answer:
To update the guestroom block from 20 to 17, access the Guestrooms section on the booking system, edit the number, and save the changes.
Step-by-step explanation:
To update the guestroom block from 20 guestrooms to 17 guestrooms, you should first access the Guestrooms related list section on the booking management system. Once on this page, locate the current entry stating the block of 20 rooms. You'll need to edit this number to reflect the new amount required by your client. This process typically involves clicking an 'edit' button or link near the guestroom block count, entering the new number of rooms (17 in this case), and saving the changes. It's essential to verify the updated details to ensure the reservation reflects the correct number of guestrooms.