You can always edit and customize .desktop launchers with a text editor (gedit for example). I do not know if you are using official Blender or installed from repositories. Suppose you use the latest official blender 2.79 and you downloaded tar.bz2 file in “/home/YOUR_USER/official_blender” (‘official_blender’ is a folder you created). The folder after unpacking tar.bz2 file inside will be “/home/YOUR_USER/official_blender/blender-2.79-linux-glibc219-x86_64”. Within that folder you open ‘blender.desktop’ file with gedit (sudo is not needed). You can see a line there:
You edit this line by:
Save changes in editor and close. Copy new ‘blender.desktop’ launcher to your Desktop.
If you leave the line “Exec=blender %f” unchanged, it will continue launching Blender version you have installed on your system from repositories if you have it so.
Some distributions/Desktops have the possibility to configure these things in graphic mode for example from the file explorer, but I do not know Ubuntu