I found an easy way to remove the outside window buttons when doing the slider replacement last week. All you need is two clamps, a small block of wood, and a rubber grommet (from a garden hose in my case). Put the grommet on the wood block and position so that the grommet is on and around the window button. Then put a clamp on each end of the wood, soak the button in a little PB Blaster, and remove the button with an appropriate tool (I used needle nose pliers inserted in the two button holes). Easy and only took a few minutes for the PB Blaster.
How to remove window buttons (slider replacement) - 
