This is a new and separate install of ET. There may be other ways to do it, but this is how I did it.
Download the following
2.55 Install
http://returntocastlewolfenstein.filefront.com/file/Enemy_Territory;143802.60 Patch
http://returntocastlewolfenstein.filefront.com/file/ET_Patch_Windows;394832.60b Patch
http://returntocastlewolfenstein.filefront.com/file/Wolfenstein_Enemy_Territory_260b_Patch_win32;620121. Extract the 2.60b patch
2. Install the 2.55 executable, "WolfET.exe", create a new directory in “My Documents” folder, don’t want to overwrite your present 2.55 installation.
3. Install the 2.60 patch executable, "ET_Patch_2.60.exe", need to direct it to your newly installed ET in “My Documents” and it is an automatic patch.
4. The tricky part. Go to the win32 folder in the extracted 2.60b folder there will be 2 executable icons. et.exe and etded.exe. These 2 icons will replace the 2 executables in the new ET Folder that was created during installation. Copy the 2 icons and paste them into the "Wolfenstein - Enemy Territory" folder which is located in the “My Documents”, NOT THE ETMAIN, but the ET folder. It will ask if you want to replace them, reply "yes to all".
You have now installed and ready to play ET 2.60b.
A few people have been running into the following problem after they patch to .6b. When they sign on to a server they get kicked by PB with the following message:
Server disconnected - Punkbuster kicked player 'SSNuke719' (for 1 minute) ...
Violation (GAME INTEGRITY) #20006
You need to update your PB. I found if you do the following it should take care of it:
In your new ET install folder go to the pb folder. Inside you'll see "pbweb.exe". Double click on it, this puts you in a dos window and runs an update program from the web. It might take 10 minutes or more so let it do it's thing until it says "press any key to exit" you should be all set to go.
Start up ET go into console and verify you have patched to 2.60b.