From what I remember ( I haven't had to change a shell plate since last summer...), the easiest way was to:
1) remove the shell plate carrier from the press.
2) remove the case ejector pin from the carrier
3) turn the shell plate until you can get something (a stubby Phillips screwdriver?) into on of the shell positions.
4) once you've got the shell plate blocked, use a 1/4" Allen wrench to unscrew the shell plate.
DON'T TURN THE SHELL PLATE CARRIER (THE DIE CAST ASSEMBLY) UPSIDE DOWN UNTIL YOU HAVE THE SHELL PLATE BACK ON!!!!!!
The Lee instructions are also pretty good at describing the process.