always download your current wordpress first. save it somewhere safe.
this is how i upgraded from 2.3.0 to 2.3.1:
download, unpack, uploaded every file owerwriting the ones on server. DONE.
Note that this is not how its recommended and it all depends on how much changes youve done to themes included in the releases. but since i use a third-party theme and not alot of plugins this hardcore upgrade works just fine.