Get a lens spanner wrench if you don not already have one. This one
http://www.ebay.com/itm/Spanner-Wre...351509?hash=item25bff5de95:g:rAcAAOSwYSlXhSV5 is very good, rigid and adaptable to many lens.
Look at each end of the lens. See the slots 180° apart? Adjust the spanner to fit these slots on the end of the lens you want to remove. Apply acetone aka nail polish remover between the ring with the slots and the outer barrel. After 5 to 10 minutes unscrew the retainer ring with the spanner. The ring(s) may be age stuck and hard to break loose.
The 3 set screws hold the aperture ring in place with one of them being a long pin that mates into the aperture operating ring inside the barrel. There is a trace amount of lube on that internal ring that will dry out and cause a hard to operate aperture. Clean with Isopropyl Alcohol (90%) then apply a trace amount of light weight grease (white lithium) to the solid sections each side of the slot. Extra fine powdered graphite can be applied to the aperture blade pivots if needed.
