From the admin dashboard click on Orders then select the fulfilled order that you'd like to edit. Look for "We successfully fulfilled 1 line item" click on this line and it will expand and give you the ability to update/edit your tracking for the fulfilled order. See screenshot below.
For anyone having trouble with editing the tracking number, or shipping method, that is using Spocket (or another fulfillment app), I had this same issue and was told the information could not be edited or updated even after adjustment on Spocket's end. However, I found a solution that was extremely simple and worked right away.
The option to edit tracking will not appear on the desktop browser if the shipping info was sent/synced to Shopify by your fulfillment app/company (i.e. Spocket), but this is what worked for me. It also updated on my browser after the following steps were completed:
Download the Shopify App (I use the iOS app).
Go to your orders, and click the order which needs the change.
Click the three dots (...) next to "Fulfilled by (Spocket)"
And click "Edit Tracking"
I don't know why this is blocked for Spocket fulfilled orders on the browser or why it is available through the app, but it appears to be the only workaround right now. Good luck all!