Open the Manage site dialog from the Tools menu and go to the Archives node. Then hover over the archive you want to modify. Three links will appear as shown above that allow the editing of archive properties, cloning of the archive, or removing of the archive altogether. When you clone an archive, the archive will be assigned a new archive id and added at the bottom of the archive list (provided that the list is sorted by archive id.)
Cloning an archive is very useful if you want to retain all archive settings and only make minor modifications to an archive's search filtering, for example.