You can use something called the Mirror App
It's actually really simple.
Not many people actually know this but screens are actually made of mirrors and PC boards with a little bit of wires and string.
You need to get a screw driver set and peel off the adhesive rubber screw protectors around the screen, they should reveal screws, remove the screws and you should be able simple just remove the screen front cover. This will reveal a lot of PC boards and the display unit, there will be screws around the display unit, all you need to do is remove those screw, lean the display unit at an angle and disconnect any wires attached to it.
You need a mirror approximately the same size as the display unit that you just removed.
Now you need to take your hot glue gun (silicon works just as well, but get the most expensive one as that ensures high quality adhesion and you don't want the wires to come off once all the screws are back in.), for each wire, place a blob of glue on the back of the mirror, carefully attach each wire to the glue, it does not matter what gets placed where, if there is a plug attached to the end of the wire then just pull it off.
You need to be very careful, as there are no screw holes for the mirror, you just need to use your hot glue or silicon to stick the mirror back in place, once you have done this, place the front cover back and all it's screws. You can throw away all left over screws, plugs and the display unit you removed.
Congratulations you have successfully installed the mirror app.