I've had a bunch of my Sapphire hd5850 fans fail, and didn't want to bother with RMAing them. Take two small screwdrivers or other prying implements and pry the fan upwards, making sure to apply force to both sides equally.
The fan hub and blades will pop up, then you can remove them, clean the shaft and add a drop or two of lightweight oil to the bearing (sewing machine oil, zoom spout, 3-in-1 for electric motors)
Push the fan hub back on and it will snap back in. You'll be good to go.
My fans failed after only 3 weeks when they were brand new - I oiled them once and they've been running for 5 months now with no problems.
Really? No need to remove screws or anything?