Saso,

To replace the valve spring, you'll have to remove the cylinder head. Place it on a small piece of wood so that the wood rests on the valves. You can grab the outside of the valve spring retainer with a small pair of needle-nose pliers. Press down, and the retainer will release the small clips.

Remove the old spring. Put in the new spring. Put the round retainer on the spring and press it down to compress the spring. Insert the smal wedges to lock it all togeter.

You have to have the valves supported so that they won't move when you press the retainer down the valve.