You need to take off the lens mount and don't forget first to remove the two retaining screws for the contact block or you will damage the contact strip from the block to the CPU located deeper inside the lens casing.
Can't remember the exact layout of the aperture collar on this lens, but usually there is a small ball bearing or spring inside, so be careful when you lift off the aperture collar so parts don't fly away. These tiny rascals can be infuriatingly hard to find later ....
Thereafter, just clean out all the dirt and grime inside and add the minimum of lubrication before you put the ring back. Finally, put the screws for the contact block in place and ensure they hold the contact block well and tight. If the block is worn or damaged, which apparently happens more easily on this lens because the block lacks the metal backing seen on later models, you can get a replacement from me.
