You need to have administrator privileges or to be a Project Manager of a project to be able to archive projects.
If you have completed a project and you would like to remove it from your Main Dashboard from preventing it getting cluttered, click on the Project Settings icon (dotted menu) of the project and select Archive Project.
Confirm the action by clicking on Archive.
You can access the archived projects in your account anytime from the Archived Projects and Boards tab of the Administration Panel.
Here you can find information on the archived projects and boards: project name, board name, the user who archived it and the date the archiving occurred.
To restore an archived project, you need to unarchive all of the boards from this project. Go to the Archived Projects and Boards tab of your Administration Panel and click on Unarchive to the right end of the boards.
Note that you can only archive projects and boards but not delete them.