I've done this on a 1998 S70 and a 2001 V70 T5a. Both were different, but the concept and the attach point mechanism is the same. There will be a petal type fixture that stays attached to the car, and the air filter fits over this. As it is pushed on these tabs compress and re-expand once through the hole. To remove, you need to re-compress the tabs. On my 98 S70, I had to do this to each tab leaf with a screw driver while keeping an upward pressure on the air box. On my V70, I found a small 16 point socket that I put on an extension, then puched down on the pedals (to compress them), and pulled the box up at the same time. Hope this gets the concept across.

Jerry
01 V70 T5a