How to remove WRX air filter?

No more screws. Just lift it out then up. It might even help to remove the intake scoop to get better access. The hard part is getting it back in. With the filter seated on the lower part of the housing, it doesn't go in easy. You need to partially put the housing back in and then slip the filter in there. Otherwise, it's near impossible to get the housing back in place.

You've got two tabs on the lid, at the end away from the fender, that slide into slots in the box. Unclip and lift that end slightly, and pull the lid towards you.

Seating the lid back into the slots is more difficult...easiest way to do that is move all the hoses out of the way, and undo the air duct at both ends (output from filter box). Be sure to plug the MAF back in when you're done.

The lid can be re-seated without detaching all that, but it's pretty tough to tell if you've got it all the way down in the slots...