You have to use the Windows Insider app, follow its steps, and then Update as normal.
Be warned: I have the same phone, and you will have problems. Don't do this unless you're happy to be testing and giving feedback to improve the OS for others. Don't do it if the loss of basic functions is going to make you angry. If your phone is important to you, you shouldn't be participating in the test, because nothing is going to be better for you than it was on 8.1 (yet).