How to install minecraft 1.8?

https://minecraft.net/...t/download

A lot of people thought that the Minecraft 1.8 Adventure Update would be showing up yesterday, September 8th. When that didn't happen, the Minecraft community breathed a collective sigh of sadness. Today, however, those blocky frowns have been turned upside down. Notch and Jens "leaked" the 1.8 pre-release onto 4chan. The build has their endorsements, as seen in the Minecraft subreddit and is safe to download and install. If you're interested in seeing a list of the changes and new features of the game, Minecraft Forums is updating a list on their front page. Perhaps the most exciting change is the introduction of Beef and Chicken, delicious!

Download the launcher, change the version to beta1.8 in the settings.