No problem at all!
You'll notice that the clear button caps are removable. Look along the top or bottom edge and you'll see a small groove. Use a fingernail or small screwdriver to 'pop' the top off each button.
You'll be left with a clear plastic cap and a white plastic insert in your hand and the 'post' of a pushbutton in the system chassis.
Using the same fingernail or screwdriver, separate the two plastic parts.
Then, use the template I've attached below to design your own button legends.
Print on regular paper, then cut to size. You'll sandwich the paper between the white insert and the clear cap.
Once the pieces are firmly attached, snap the button cap assembly back on to the button post and enjoy.
John