A phone currently running Windows Phone 8.1
A Microsoft account and Internet access
Now, while installing it on Lumia 730, the first thing I came across was the option of Insider Fast and Insider slow, which is what we call as Fast Ring and Slow Ring on Window 10 Desktop. Insider Fast is the bleeding edge and you are first to get preview build, while insider slow is when bugs from Insider fast are sorted out and you get the Build later.
How to update:
Once you pass this screen after selecting you option your phone gets configured and reboots. Then you need to go to Settings—>Phone Update and check for updates.