To be honest I've no idea as I've never done the filter myself. I do believe its under that panel you speak of but I've never done the job (I know, I'm a saff nut and I should have but I've had other hassles with the safrane to worry about). You could post over at volvoforums.org.uk and see what they can advise - it's the volvo 850 engine forum you want - be sure to mention its the AW 4 speed box and someone will be able to help
The only thing I can say about changing the fluid is that there is a sump plug at the bottom of the box that you drain the fluid out of. Make sure you have two large 6L empty water bottles though as you'll need them. The reason I say this is because Ren-Oh give you no measurements - they simply say to replace the fluid with the same amount that came out
When emptying out, catch the fluid in your tray, then empty into one of the empty bottles. Then fill the other bottle with the fresh stuff (Dexron2E or Dexron3) to the same level (ok you could go 1mm higher to allow for loss in the tray etc). Stick a funnel in the end of the ATF dipstick and gradually pour it in nice and gently. Remember, the engine must be OFF.
Once you've done that, you can run the engine (shift through the gears slowly with your foot on the brake a few times) however be warned that this only replaces about 1/2 the ATF that was in the box so you're best off running it around for a week or two to let the ATF circulate and dilute the older sludge before doing the entire process again.