How to replace filter things?? Really confused?

ok im new to fish keeping i have a 10 gallons 1 gupy 1 black molly 1 sunburst platy. i cycled the tank. the tank is 3 months old. i change the filter cartidge every 5 weeks.(is that what its called its the white thing in the middle that is held by the frame. ihave a whisper 10 filter. i dont think im doing it right because i just had an ammonia spike up to 1ppm after i changed the filter caritdge. should i soak the cartidge in the tank so it gets the bacteria. if i change the caridge doesnt the bacteria have to recolonize. theres a sponge in the filter frame thing that came with the filter caridge. does the bacteria stay on that. i dont want my fish to die plz help. how do i properly replace the cartidge