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.
Download Links:
Cross-platform server jar: https://minecraftlucky.com/dowload/file-3565
Cross-platform server windows exe: https://minecraftlucky.com/dowload/file-3566