How do I change my keyboard shortcuts in xubuntu?

Keyboard shortcuts are tucked away in your Settings Manager - Window Manager - Keyboard

Scroll down the list - you will see several workspace shortcuts, some defined with a shortcut and some awaiting to be defined.

There are more available in Settings Editor in Settngs Manager, which can be launched from the command-line using:

xfce4-settings-editor

This will display:

For Xfce 4.8 (Xubuntu 12.04 LTS)

Keyboard shortcuts are accessible from Settings -> Settings Manager -> Keyboard -> Application Shortcuts:

• Choose "Add" to add a command you need to assign to a shortcut.
  

• In the window following "Ok" press the key combination you want to assign: