How to Assign Custom Keyboard Shortcuts in Photoshop CS5
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Photoshop provides a robust menu for changing keyboard shortcuts, which is accessible in the Edit menu:

You can also use the default keyboard shortcut Alt-Shift-Ctrl-K to access the keyboard shortcuts menu. That is a keyboard shortcut you probably don't need right there, unless you intend to be changing shortcuts often.
The keyboard shortcuts menu is pretty self-explanatory. The menus are listed as they appear in the top of the screen, and you can arrow through them and assign new shortcuts as needed:

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