I would
only do the back first, the front is bit of a minefield... literally! Concentrate around the ANA chip area.
Make sure you remove the heatsinks before doing any of this otherwise your board is going to DIE!!!
Put your heat gun on low/300C and heat the board in small circular motions, always keep the gun moving otherwise things will pop off! Then put the gun on medium/400C-500C and do the same for 3 minutes... you may see a little smoke, at this point stop as the solder has melted (which is what we are trying to achieve). Let the board cool for around 45 minutes before testing it out, also, make sure there isn't too much thermal paste on the heat sinks.
If it still fails, all is not lost. Do the top of the board but this requires a lot of preping so we'll come on to that later if need be.
Hope that helps
I can't fret enough how important it is you read up on how to do the top before even thinking about it, if done wrong you can pop a capacitor and believe me, you don't want to be around when those things go off!!!!! (even the small ones)