First thing is to freeze or physically write protect the tape, so that other images don't expire and allow the tape to be re-used.
Once you've done that you need to read up about "importing expired media". It's a two stage process, and I won't repeat the manuals here.
Have you already tried and hit a specific problem?
If you do not know the "import process" I suggest that search through manuals, as it's covered and mentioned in several places.
You can download docs for you O/S and NetBackup versions here: