“Basically I hold the LED board in a vice with little wooden soft jaws I made up to provide insulation. I use a hot air attachment on a gas powered soldering iron (not a gas torch) and heat the underside of the board until I see the lead flow (melt). Then I gently press down on the dome with a concave ground screwdriver and hold it for a few seconds, making sure not to move the LED. This lifts bottom of the dome around the four sides of the emitter surface as well as testing if the dome is soft enough; if not soft enough I apply a little more heat.
Once soft enough (and the solder has taken again) I lift the dome off by popping a screwdriver under the edge of the dome on the opposite side to the fine gold wires. Normally the dome will just pop off with a little pressure. If the dome does not soften enough or it is reluctant to pop off I soak in petrol. The heating and lifting of the edges speeds this process any way.
Similar to this but it pops off a lot quicker and cleaner.