How do I care for the moss filled topiary? can I replant moss when it dies?

The moss is a Dried Green Dyed Sphagnum moss.   Dried moss is actually moss in a dormant phase.  If you plant to plant in it, you would just take care of the plant of your choosing by following the growing instructions of that particular species of plant.  If you don't plant in it, there is really not much care needed for the topiary.  Keeping it out of direct constant sunlight will help avoid fading of the dye.  You can certainly refill the topiary with moss if in future years it needs replacing.