One way by changing the values in the registry but this tool eliminates the need of doing that and its called SoftKey Manager. Its a freeware SoftKey Editor for Windows Mobile, allowing you to change the defaults of your Today screen softkeys with just nice little GUI interface.

Features
* Set Today Screen Left and Right SoftKey Names and Shortcuts.
* Easily set a SoftKey to anything in your Start Menu.
* Easily set a SoftKey to any file on your device.
* Easily set a SoftKey to an Internet Explorer Favorite.
* Reset to Defaults.

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