1. Click on the Trip
This will open the Trip View.
2. Click on the Itinerary/Proposal
Click on the itinerary/proposal on the trip you'd like to archive.
3. Click on Settings
4. Click below Type
5. Change to Proposal/Itinerary
Just select which you'd like!
Pro Tip: If you wish to keep a Proposal or Itinerary version you can choose the Copy option to create a new version of that trip and then convert it to the Itinerary or Proposal on the copied version.
For more information on Proposals and how they work, click here.