How do I check or reset the linking of Quicken to Fidelity?

First, I would wait a few days to make sure that the transaction doesn't eventually get posted before you go deactivating and reactivating.

But, if you decide to do such, then go under that account's settings, and click on the Troubleshooting button. Click on Deactivate Downloads to deactivate.

You would then have to set the account up again by Clicking to Setup Transaction Download and walk through linking this Fidelity account.