(Avoid this type unless you're conscientious about cleaning it.) To replace old outdoor vent covers, start by pulling apart the duct at the last joint inside the house. If they don't pull apart ...

Then, use a screwdriver to remove the vent covers. If they’re especially dirty, take them outside or to a sink for a deeper clean. Soak the vent covers in warm, soapy water for about 15 minutes ...