This should work on Ubuntu 15.10:

• Install compizconfig-settings-manager Install compizconfig-settings-manager.
• Press Alt-F2, type about:config and press Enter.
• Select the "Launcher" tab, and change the keyboard shortcut for "Key to show the Dash, Launcher and Help Overlay".

Quelle: http://askubuntu.com/questions/144783/how-do-i-change-the-shortcut-key-to-launch-the-dash