How do I reset my QR code design to the default?

In the QR Code Designer, pick the Classic preset. That restores the plain black-and-white, square-on-square look. Save the link and the QR is back to standard.

If you've been iterating in the Custom tab and want to clear your saved custom design, open Custom and click Reset at the top right of the panel. That wipes the locally remembered design so the next time you open Custom you start from the default rather than from your previous tweaks. See Can I reuse my QR code design for other links? for how that persistence works.