Delid with vice and hair dryer method, https://www.youtube.com/watch?v=PGJ4YkEHx34
easy and almost risk free, just get the orientation of the CPU the same as in the video. Close the vice with one jaw on the IHS, and the other on the CPU Substrate. Don't use too much tension and make sure it is almost perpendicular to the vice to avoid it twisting. Heat it with hair dryer for a minute, tighten vice slightly and it will "pop" as the glue melts and the IHS moves a few mm, don't over tighten the vice or you may damage the capacitors on the CPU under the IHS, just wait for the "pop" then pry the IHS off with your fingers, heat it a little more first if necessary.
Apply a very thin layer of GC Extreme on the die and on the IHS and you are good to go. This will net you an easy 5-10*c, maybe more.
If you want to go further you can lap the IHS with 800-3000 grit sandpaper to remove the concave (hole) in the IHS that most Intel CPU's have, you'll also need to mildly lap your heat sink to remove any bow that it may have to compensate for the concave IHS, some manufacturers do this as they know the IHS won't be perfectly flat. You can also mildly lap the bottom edge of the IHS to get a thinner layer of TIM between the die and the IHS by allowing the IHS to sit lower on the substrate. If you go to the trouble of doing all this, and it is at least a days work, then you should also use CLU or the upcoming Thermal Grizzly liquid metal TIM, I forget the name, cosmonaut or something like that. This will net you a 10-20*c drop in temps. I'd only go to this much trouble if you have a particularly good CPU and want to overclock quite high, otherwise a simple delid and relid with GC extreme is fine.