I had the same difficulty at first, right pain in the rear!
You have to push in the little black button at the back and kind of push the blade back and out to get it off.
To get the new blade on, get a blunt knife to gently push out the metal arm (just looks like a bit of metal where the blade sits). Once the arm is out you can slide the new blade on and in place. To get it in position once it's sort of down I found through various internet searches that if you turn it on and push the blade and unit together it clicks into place.
Very difficult to describe and the instructions were useless!
However once you've figured it out once it's really easy.
Turn the clipper over so that the underside of the blade is facing you. With the pads of your thumbs (one on each side) press the blade away from you (it might be stiff at first), it will flick out with a little "thunk" as it comes to rest in the open position - don't worry, it's normal). The blade will then slide off the tab. While the tab is in the open position, slide the new blade on, and click back into place.
The spring is quite strong on the small tab, so it if flicks down while you don't have a blade in, you will need a blunt knife to wedge in behind it to flick it open again.
Thanks all! I managed to get them off and the new one on I've also managed a rather bad first clip job. Luckily we're not leaving the yard any time soon!