Using key bindings editor you can change a shortcut for almost every action in TuxGuitar. It allows you to view and change default keybindings, as well to add new ones.
Key bindings editor dialog is accessed through Tools→Shortcuts menu.
The editor is a table you can modify. In the left column is the name and type of the action, and in the right one is the shortcut key combination. If you double-click on the combination cell, you will get the chance to insert or change the shortcut, or delete it with Clear button.
By clicking the Default button all of your modifications are reverted to the default keybindings for TuxGuitar. You can see the list of default keybindings.