If you mean there's a bit of lateral play in the cassette on the freehub then that should disappear when you've torqued up the lockring. Is that right?
To tighten up the freehub body you will need to remove the axle in order to get a very large (10mm-12mm depending upon hub make) allen key in to do so. Usually this is inserted via drive side but some it is from non-drive side. If it is already tight then the little tiny bearings in the freehub body are knackered and you'll need to replace the freehub body - you'd need to identify which fitting it is as there are several different ones.
Sorry, misunderstood what you were getting at. lpretro has given you the chapter and verse answer.