Welcome
This should be in the New Members' Forum; a mod or admin will probably move it soon, so don't worry about reposting
It depends what you want to keep. As a rule, to upgrade, overwrite the CGI scripts, the Templates directory and the Wordlets files (NOT the variable files). This will mean your members, posts/forums and current settings stay intact.
If you could provide a little more information, I could give you more specific details regarding what to do