you have to remove this spring which is located under the aperture ring:
Before disassembling the lens, please watch the aperture ring carefully: depending on the lens model, it may have two small screws which are close together. in these lens types there's a internal lever which connects the iris system to the aperture ring and is attached with these screws: you have to remove them before trying to lift the aperture ring off. Reassembling the ring may be a nightmare because the lever moves quite freely and can be difficult to hold on place when attaching the screws back: I recommend using small paper stripes under the lever so you can somewhat hold the lever at correct position so you can get the screws back properly.
After removing the click spring, you may have to add more friction to the aperture ring. One way is to use small tape stripes under the ring and add couple of them over each other to get desired friction.