Before reflowing, be sure to remove the plastic centering ring around the old emitter. The best way to reflowing is to gradually bring the emitter up to temperature like with a cheap electric hot plate. Some have used small torches to rapidly heat the MCPCB up, but that’s not really ideal. Be sure you have liquid solder for the new emitter as well as new thermal paste when you go into removing and replacing the MCPCB, the board with the emitter on it.

It looks like you can get that emitter on a 10mm-16mm MCPCB, if you don’t feel comfortable with reflowing, you can buy on already on a board with the same dimensions as the old and do a straight forward swap.