To archive a form, go to the Forms page, locate the form you want to archive, and click the "Options" button. A side panel will open, displaying the "Archive This Form" button.
When you click on that, a confirmation dialog box will appear. Clicking the yellow archive button will confirm the action.
To view and retrieve your archived forms you just need to go to the forms page, click on filter and then Display Archived
The archived forms will now show on the main forms page. To unarchive you just need to click on the lock symbol and the form will then be live again.