http://manual.b2evolution.net/Move_my_installation might help, and searching the forums most certainly will. It's not hard, but it's not exactly obvious either.

Backup everything twice, know the steps you're going to take, make sure each step seems to have done what you wanted it to do. That's the short version ;)

also looking at the FAQ section might help first.. for such common questions..

like: [url=http://forums.b2evolution.net/viewtopic.php?t=1137]Moving your b2evolution installation[/url]