Firstly, if an old repair has failed, it's usually best to remove it and start again rather than just stick another patch on top. If a patch is really badly stuck, sometimes you can just peel them off by hand, but if there's a fair bit of resistance just pulling at it is likely to either end up with you loosing grip (potentially hitting yourself in the face...!!) or
damaging the PVC or hypalon fabric in the process by delaminating it.
So, to answer the question - a heat gun (or hair dryer) is needed. Warm the patch with the heat gun, but don't get too close and do keep the heat gun moving as this avoids concentrating the heat too much and potentially blistering the good material. If the patch is already lifting, use a pair of pliers to grip and gently peel back the patch. Pliers avoids having to grip thin fabric and keeps your fingers further from the heat, so helps avoid burning your fingers - ouch!
But what if the patch doesn't already have a lift? If the patch hasn't got a nicely peeling corner or edge, then you need to create one. The best way to do this, is inflate the tube or inflatable to normal, or just above, pressure. Then once the patch has been warmed, use a BLUNT scraper to lift a corner. This method can still be used if the inflatable isn't air tight, but there is more chance of damaging the fabric and putting a hole in it if isn't well inflated.
We will be doing a proper how to video of this soon.