Is it simply a "collapsed" lifter or has it cracked or broken? If it has collapsed, I would consider replacing all of them while you are at it. Pulling the intake and valve covers should get you access. You will need to loosen the rockers to get the push rod out and then the lifter should come out easily with a magnet. Check the bottoms for excess wear where they contact the cam. Also, could the problem actually be a flat cam lobe?
SWMBO's '66 Coupe 289, 2-V, C-4(Pony Interior, Rally Pac, A/C & Tiffany Stripe) & An absolutely rust free '65 289-2V C-4 coupe project. Now almost done. Only took 13+ years.