I'm sure it's harder than you might think... If you haven't pulled one apart, when you slide the circuit board out of the top, you'd see the LED inside is a multi segment 2 digit LED display, soldered onto the board.
Not as easy as simply popping out wone light and inserting another. It's got somewhere along the lines of 20 solder joints that would have to be de-soldered, and solder in a new display...
I wouldn't mess with it unless you're very confident with your soldering abilities...