You need a reballing kit, but I really dont recommend you go anywhere near one if you have no experience using one, it involves removing the gpu from the motherboard, applying new good solder to all the points, and soldering it back on, you will need a hot air gun to remove the gpu. It really is not an easy task. I would recommend getting it done by a pro.
Have you tried to heat the board using a hot air gun then carefully while its hot, replace the gpu heatsink on the board, and apply light pressure to force the gpu down, hold it there for a few minutes until it has cooled down a little, then remove the heatsink. I did this to a friends 360 after it died on him, we tried the x-clamp fix, coins under ram modules, but it didn't work, this however solved the problem, its been working for around 4 months now. Not sure how long it will go, he has a launch console.
