The simple solution is to open the files in the backup folder with Notepad++, possibly doing a Save As or Rename to put them in a different location after opening.
Ideally, if you had also backed up the session.xml file in AppData\Roaming\Notepad++, you could theoretically restore that file also and have it restore your session. However, it’s very finicky, and if any small detail has changed (path, for example) for any file, it will simply not open that file in the session. So the more reliable solution is the simple one above.