Note from Cyan : Since PokeAcer is banned and deleted his website's guide, here is a new one from Lacius.
Updating to 5.5.1 or 5.5.2 from lower versions
Guide from Lacius
- Your Wii U needs to be on v5.3.2, v5.4.0, v5.5.0, or even v5.5.1.
- This guide will update your console to v5.5.2 but will update/keep the browser version to v5.5.1 to retain the browser exploit.
- Do not disable your DNS filtering after updating, or the browser will update itself to the latest version!...
Updating to 5.5.1 or 5.5.2 from lower versions