If you find that you are running a FUDforum release older then 1.2.8 you will need to upgrade your forum to version 1.2.8 before you can perform any other upgrades.
This upgrade script is available from the FUDforum's download page in 2 compression formats: bzip2 and gzip.
The download links for the upgrade scripts are: update-v1.2.8.php.tar.bz2 update-v1.2.8.php.tar.gz
Now that you have the upgrade script on your server, decompress it and place it inside your forum's main web directory, so that you can access the upgrade script by going to http://you_forum_url/update-v1.2.8.php
Now that the upgrade script has been positioned, open the http://you_forum_url/update-v1.2.8.php URL with your web browser. You will then need to enter the full path on the disk where the GLOBALS.php file can be found. Once you've entered the forum submit the form, which will begin the upgrade process. The script will output various progress indicators, letting you know of what is it doing at the moment.
If you have output buffering enabled or using buffering modules such as mod_gzip you may not see any output of the upgrade script until it finishes.
Upon the completion of the upgrade script, you will need to run a consistency checker to rebuilt various caches and verify consistency of your forum's data. The upgrade script will try to automatically to run the consistency checker by popping up a new window using JavaScript. If for some reason such window does not appear click on the 'MUST click here' link, which will execute the consistency checker in the current window.
Once the consistency checker finishes running, the upgrade process is complete. The only thing left to do, is to remove the update-v1.2.8.php script you've placed inside the forum's web directory in the 1st step.
Once you've upgraded to version 1.2.8 you can now upgrade to the 2X series FUDforum.