You may use a little hack to change the displayed version number. Short indication to do it is to change the shared library file by replacing every occurrence of your old version number with the new one:
sudo sed -i -e 's/your_current_version/desired_version/g' /usr/lib/flashplugin-installer/libflashplayer.so
After this, restart your browser and go to Adobe flash "about" page to see if it worked.
Longer indications may be found in this blog post.
edit: I should add a word of caution here: it is a very dirty method, which will likely work only until the next update of your flash player. And it will likely work only as long as the original and desired versions number have the same number of digits ! So, please do a backup of the original file before attempting it, i.e.
sudo cp /usr/lib/flashplugin-installer/libflashplayer.so /usr/lib/flashplugin-installer/libflashplayer.so.backup
and to restore the original unaltered file, type:
sudo mv /usr/lib/flashplugin-installer/libflashplayer.so.backup /usr/lib/flashplugin-installer/libflashplayer.so