how to rank down on halo 3?

ok so the thing is there is an achievement for getting 5 exp,and i have 10.I saw some videos on youtube with people having the same problem,and apparently the only way to fix it is to rank down to 4 exp and get 5 again.now my problem is idk how to rank down lol.rep+

Ask on B-net.

It worked.rank down to (i did 0 but anything below 5 should work) and then rank back up to 5 exp.you should re-receive the graduate pin and this time unlock an achievement