The valve spring is held in place with a cap, in the center there are two retaining clips which creat an interference when the lock into the top of the stem itself. There are actually two springs, and inner and outer, belwo is a shot fo the cap, springs.
Pics of retainers on the end of a telescoping magnet:
In order to remove the spring, you need a spring compressor as pictured below: The top part of the compressor which looks like a pick presses against the valve face, the bottom of the compressor uses an attachment to push on the outside fo the valve spring cap. This allows you remove the retainers and release the tool allowing you to take out the valve
