In order to download the WM6 ROM update for your HTC P3300 you need to be registered in the HTC e-Club.
WARNING: All your data will be erased in the update including personal data such as Contacts, Calendar appointments and also your installed applications. Use Active Sync to backup Contacts, Calendar, E-Mails, Notes, etc... Copy your personal files such as photos to the PC also. Please note that Active Sync doesn't backup your SMS messages you must use third party software.
Before installing additional software you will probably want to do a simple tweak. This ROM comes with a application lock that doesn't allow you install unsigned CABs. To remove this lock you only need to run the App Unlock utility and the lock will be gone. You can find this utility here.
One of the things I wanted from this update was the VoIP feature. After the installation I discovered that this feature wasn't available. This can be solved with a couple of CAB files. The first one, WM6VoIP.CAB, will install the VoIP features. The second one, SetupVoIP, is an utility to help you configure the SIP server of your preferred SIP provider, you can download it from here.
If you like the black look of the new HTC Phones you may want to bring that fancy new look to your HTC P3300.
This information was gathered while updating my HTC P3300. Most of it was found in the xda-developers forums, a great place for HTC owners.
All of the above was tested with sucess in my HTC P3300.
I will continue to update this site with new tweaks that I may apply to my mobile.
For suggestions/questions fell free to contact me here.