You shouldn't need to replace anything. The only time you need to replace filter media is when it is literally disintegrating. Assuming these are the small stone balls and you really really want to do something to them; you could try soaking 1/4 of a tray at a time in bleach to burn off all of the organic residue, rinsing it thoroughly, letting it dry for a few days, and then soaking it in water with a large dose of prime. But honestly even that is unnecessary.
Just wash your media in old tank water and it'll be fine.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor