Updating Internet Explorer

I use Firefox but need to know how to update to the latest version of IE. I currently have 6.0, but have been denied access to a site because I am told that 5.0 or higher is required. I don't use IE, but would like to have the latest version just on general principle, although if 6 doesn't work despite being an update of 5, maybe 7 wouldn't do the job either...

(I can access this particular site indirectly by using IE instead of Firefox, but I don't like being limited in this way.)