want to help father-in-law

My name is Peter. My father-in-law (almost 82 years young), just bought a 1998 Chevy silverado. He needs to set the compass on the mirror. He doesn't have an owners manual, so he asked be to help. I figured I'd go to a forum and ask the experts! I live in Maine. Can anyone out there provide me with simple directions for this one task? Thanks in advance.