In the Planner 5D Windows app, projects can be archived, which essentially removes them from your immediate view, but doesn't permanently delete the data. Here are the steps to archive a project:
Open the Planner 5D app.
Navigate to the 'My Projects' section.
Locate the project you wish to archive, then click on the three dots icon in the lower right corner of the project's thumbnail.
Click on the "Delete" option.
Your project will now be archived and removed from your active project list.