Apparently someone leaked the install files for the NXE on the internet and it is now available for all to try out. Now just to let you all know, YOU CAN NOT CONNECT ONLINE WITH IT TILL NOVEMBER 19th, it will not allow you, so if you plan to game, do not use this.
Also there is no guarantee that Microsoft will not ban your console or get angry at you, so USE AT YOUR OWN RISK.
Now to install this, it is really simple, all you have to do is download the update files, you can get them http://digiex.net/homebrew/594-xbox-360-nxe-preview-dashboard-exclusive-early-download.html at digiex.net. After you have the file, you just put it on the root of a USB drive, slap that bad boy into your Xbox 360 and turn it on. From there you will be asked if you want to update and now your off and running. Again, use at your own risk.
What Microsoft has said about this:
The Xbox 360 Fall Flash update will be available to you by 8pm PST on October 30th (11pm EST on October 30th, 3am GMT on October 31st).
While exploring the New Xbox Experience (NXE), you may come across bugs or have some suggestions. Please share those with us by going to Microsoft Connect and clicking on the Feedback link.
If you would like to post comments regarding your experiences with the New Xbox Experience, or interact with other preview participants please use the Newsgroups link on the connect site. Although we are monitoring the newsgroups, all bugs and suggestions should be filed using the Feedback link.
If you need technical assistance with the Xbox 360 Fall Flash and you're in the US or Canada and speak English; you can call 877-XXX-XXXX. Phone lines will be open between the hours of 8am and 6pm CST. Please do not publish or share this phone number with people that are not participating in the Preview Program
In participation with this program, you should not move your storage device (hard drive, or memory unit if you do not have a hard drive) to any other console as it will also update that console. If an unregistered console is updated with the NXE update, that console will not be able to connect to Xbox LIVE until the NXE has been officially released on November 19th.
We thank you for your participation and hope that you enjoy the New Xbox Experience.