Hi George! Great question! You have a couple of different options depending on how your account is setup. Here are a few articles found in the Pruvan Support Center that will help you out:
If your projects are being imported via a client integration, then a full import will clean up and remove any older work orders that you no longer need. https://pruvan.zendesk.com/hc/en-us/articles/205695359-How-to-do-a-Full-Import-on-a-Client-Integration
You also have the option to mark completed projects as "closed". This will remove those projects from any devices and from your main project page. You can always refer back to closed projects by clicking on the "Show Closed Projects" option found on the Projects Page.
Hope this helps! Let us know if you have any other questions.