If you are running Slimserver on Windows and you upgraded over the 6.1.1 changing the firmware version is as easy as editing the squeezebox2.version file. Make a backup copy first! Just in case something goes wrong...
The default install on Windows puts this file in:
As Jakepunk stated, just change all instaces of 26 to 15. Of note, this file has a UNIX format and will look odd if opened in Windows Notepad.
To make life easier, I have posted it at: http://home.comcast.net/~mikegalusha/squeezebox2/squeezebox2.version.15
To use it, make a copy of your original as squeezebox2.version.26 and save the new version as squeezebox2.version
This assumes you did an upgrade and your firmware directory contians the squeezebox2_15.bin file. If not, you can download the 6.1.1 software from slimdevices and extract it.
I had to power cycle my SB2 to have it notice and download the new (old) firmware.
Hope this helps.