It's really quite easy, all you need is something long (40mm min) and thin to fit down into those holes. I used a 2.5mm allen key. See pic below.
Simply push your long thin thing into the hole (one at a time) until it stops then simply push a little further and it should release the clips.
NOTE you don't need a lot of force!
I recommend wrapping some tape around your long thin thing 35mm up from the end. If the tape goes into the hole past the front of the radio then you've gone too far!
When the clips have been released then you can remove the radio by putting a couple of fingers inside the tape slot and pulling on the unit. Again not a lot of force is required, if you've released the clips properly then the unit should just slide out.
*-jester-*