1. The platform is static sensitive - you should make sure that your workplace has a conductive surface that is properly grounded to avoid static voltage build-up. Static discharge while touching components will positively destroy the board.

2. You need at least a good fine potential-free electronic solder iron. Or in the worst case a gas-powered solderer to do the most of the modifications described here. And at least average solder skills.
   However: None of the modifications that require soldering operations are of very high complexity or over-average difficulty range (Except where otherwise noted).
   You will need quiet hands nontheless.

3. Rethink again about the value of the platform. If you are in doubt that your skills will suffice and are a bit worried remembering how much care and money it may cost to get another P-90 platform ... leave it.
   Or try to get someone else to do it.
   The reworks are not very difficult but before you ruin a good working platform: Think. And think again.
   The modifications are mainly easy reversible for the sake of the originalist freaks.

4. The usual disclaimer: You are doing it on your own risk. The maintainer is not responsible for any damages or losses, neither as direct or indirect consequence.
   Or in simple words: if you louse it up - it's your problem !

The last BIOS for the Type-4 platform was Revision 10. It was designed to solve some problems for machines running OS/2. The Revision 09 never really made it into the public, since it contained some serious bugs and was pretty soon superseeded by the 10 version. This however has some downsides with faster processors. It works fine with e.g. a 120 MHz clocked CPU (a P-133 set to 1:2 bus / core ratio at 60 MHz base clock) but faster combinations often end up in a 0129 xxxx error. I don't know exactly what BIOS change caused this misbehaviour, but it is a known fact. Obviously the Revision 10 alters some settings for the cache controller that are not really well thought out.

Therefore I would suggest to stay with the BIOS revision 08, which had prooved to run reliably with the Pentium Overdrive 200 (PODP200) as well as with a souped-up platform with the "Classical Pentium" 200 or even a MMX interposer and - at least - a Pentium MMX running at 180 MHz with the unmodified 60 MHz base clock.

233 MHz with a modified 66 MHz base clock is difficult. Most likely the board has too little time to synchronize between cache memory and processor core and accomplishes severe mis-hits and the system crashes with cache errors / processorboard errors of the 0129 xxxx family.

While BIOS updates are a sensible and vital operation I would recommend to upgrade / downgrade the platform BIOS before any processor or clock rate changes. Particularly altering the clock from 60 to 66 MHz might be quite hard to digest for the board and it may - or may not - fail to properly get the new BIOS version installed. If it fails there is a good chance to render the platform non-functional !

There are no known ways to re-install a working BIOS flash from outside or restore the old version or do a "desaster recovery" as known from modern boards. The platforms' BIOS chips are soldered and cannot be removed for reprogramming in an Eprommer.