Air Purifier Subscription Question

I have a 3 month subscription. Where in do I go to to change to a 6 month subscription? And how do I know when to change my filter. I have a note up on top of my app that’s letting me know it’s close to change my filter. I’ve read on other post here about vacuuming/air blow to try and extend the filters life. I usually go by looking through the filter and if you can still see through it it’s still good.