The armature comes off after you remove the nut and plastic washer. The nut should be torqued to 195 inch lbs. when it goes back together.
If you need to remove the guts of the valve JD calls for a special socket. I have not needed to remove mine so I can't provide any additional insight on the process.
This proceedure, as well as the rest of your 425 service proceedures are covered in a TM-517 Technical Manual. It's worth the money if you plan on other projects.