I could tell from the smell while printing ABS that the carbon filter was no longer filtering as much, so it was time to refresh it by leaving it out in the sun for a few hours.
I removed the lid from the filter box and tried to remove the carbon pack.
I could not get the carbon pack back out around the rail at the back of the printer. After quite some work, I finally decided to unscrew four screws at the back that looked like they might be how the box was held in place.
One of the four screws had a stripped socket, and the only way I got it out was an ez-out.
Support later confirmed that they expect you to unscrew the box each time you charge the carbon filter. The screws are self-tapping plastic screws, not machine screws; they do not engage with permanent threads. Support didn’t offer to replace the defective screw, saying only “It’s not a problem to use it with only three screws.” (Edit: later, support offered to send new threads and a new box if there was a problem using it with only three screws.)
It looks like the filter box could instead be screwed to the outside of the printer, enabling filter maintenance more easily. 
Edit: Except that the provided screws are too short for that.
I ended up designing a replacement.