Today, you can now install Windows Phone 8.1 on your phone through the Windows Phone Preview for Developers and try out a lot of the awesomeness we announced two weeks ago at Build. To do this, follow the instructions here on the Dev Center. I suggest reading the important notes on the instructions page before opting your phone in to receive Windows Phone 8.1. For more details, head on over and read this blog post from Cliff Simpkins.
For developers, we also announced that today the Dev Center is now open for now open for Windows Phone 8.1 and universal Windows app submissions.