First, find the Person you want to Unarchive.
Click Here for more Information on how to find a Person you Archived.
On the Archived Person's Page.
Click on the Gear Icon in the Information Section on the left side of the page.
A menu will open.
Click on the Unarchive Person item.
A window will open.
Click on the UNARCHIVE button near the bottom right corner.
Now this Person will be treated like a normal Person in your Farm.