The point of this parameter is to help you estimate, in very rough terms, the percent of your backfile that will remain available after a cancellation.
However, we recommend that you upload exact dates at the journal level, so we can calibrate the journal-level models more accurately. This makes your cancellation forecast more accurate overall.
If you see that this parameter is greyed out, it's because you've already uploaded journal-level dates in the Publisher Setup tab. That's good! We're using your more accurate data in the model now, and so there's no need for the less accurate "Backfile available as perpetual access" estimate.
If you want to modify perpetual access dates, you can do it from the Publisher Setup tab, where you'll be able to delete and re-upload the file with your custom journal-level date ranges.