How to ask the guests to resubmit the rental agreement?

Open the reservation from Messages or Reservations, then find the Rental Agreement panel in the reservation details. The Rental Agreement panel is its own section on the reservation, between Booking and Pricing.

If your guest has not signed yet, click Submission link. If they have signed once already and you need a fresh copy, the same button reads Re-Submission link.

Either one opens a dialog where you can copy the link or send it straight to your guest as a message. If your agreement includes an ID upload box, the same link collects the guest's photo ID at the same time.