You can NOT downgrade the Firmware. BTW, why do you want to do that ? If you are facing any issues after the update then post them on appropriate board on this Forum and you should get the solutions to resolve them. I have updated my Nokia 5800xm to the latest version since more than 6 months now and do not face any problem..
As mentioned earlier you can't downgrade. Though its your own choice / liking, the latest version has a better browser and the Maps 3.06 is embedded in the Firmware...so a bit surprised to know your decission. In fact the users still awaiting to get the update are very much upset and getting restless...