The 1.8 update is now published to the launcher! Your game should be updated automatically.
Because this is important, let me repeat that last sentence:
Please make sure that you are in the correct server mode (online mode/offline mode) when you start the 1.8 server for the first time, because profiles are not compatible between these two server modes.
Cross-platform server windows exe:https://s3.amazonaws.com/Minecraft.Download/versions/1.8/minecraft_server.1.8.exe