Sign up for the Windows Insider Program, if you haven't already.
Find out if Insider Preview is right for you, and make sure your phone is on the list of supported phones. Prerelease software doesn't always work as expected, and could cause your phone to stop working permanently.
Open this webpage from your phone, and then click Download the Windows Insider app. You can also search for the Windows Insider app in the Store from your phone.
Once it finishes downloading, open the Windows Insider app on your phone and follow the instructions to install Insider Preview. You'll have the option to choose whether you want to get new builds fast or slow. Choose Fast to get updates as soon as they're available, or choose Slow to wait until builds are a little more stable before you update.