On your PC, make a new profile, then navigate to your Anki folder \ profile name subfolder, and replace the collection.anki2 file with your backup file. Then open Anki and change to that profile. From there you can just export to apkg.
If it crashes when you try to open the profile (this will happen if you didn't shut down AnkiDroid properly before copying the backup to your PC) you might need to open the anki2 file with
sqlitebrowser and change the "journal mode" (under "Edit Pragmas" tab) from "WAL" to "delete"