(adsbygoogle = window.adsbygoogle || []).push({}); How do I go from my 4.21 Rex back to 3.55.3 Rex?

Do I just do a system update with the 999 downgrader?

You just answered your own question. After you install 9.99 downgrader through XMB, you need to install Toggle QA and run it, then install 3.55.3 through recovery menu.

Get Rebug's downloads from here: You must login or register to view this content. and Toggle QA from here: You must login or register to view this content.