It's not that difficult at all. There is a little metal tab on the top of the lens mount inside the camera, if you push or pull or move it the screen will fall out and land on the mirror of the camera. if you push the screen up it will click back in and stay there.
There's a little plastic tab on the screen that you're supposed to use to handle the screen (with scissors, atleast that's why pentax gives you a free pair when you buy one of their screens)
I'm not sure how much brighter a Beattie screen would be.. The P645 is already pretty bright but let us know how it goes!