Use your fingers... I "flick" them out easily (once you get the technique) and then separate the 2 parts of the button & insert the label. Sit it back in place (there a correct way round) and push it gently until it re-seats. You shouldn't have to use much force at all for any of this.
To change the User buttons the menu path is MENU > User > NEXT > UseSel Buttns. Use the Button/RowBtn knob to select the user button you want to set up and Type and Index knobs to select the function for the user button
#Carbonite