Hello, LB121.
When going to your Sync with Square screen (Apps>My Apps), you should have the Action button on the right side. Once you click this button, you'll see Setting from the drop down option. You'll need to select Setting so you'll be routed to Review Your Sync with Square Settings where you'll see the Edit link next to Defaults for Unmatched Items.
However, if you don't have the Action button, I'd recommend opening your QuickBooks on a private window. Here's how:
a. For Chrome, press Ctrl+Shift+N.
b. For Internet Explorer or Firefox, press Ctrl+Shift+P.
c. For Safari, hold down Command+Shift+N.
If everything works fine on a private browser, i'd recommend clearing your browser's cache or switch to a different one.
Let me now if you have other questions about QuickBooks. I'll be around to help.