With this update, you now have an option of logging in with your Microsoft account too and no point for guessing that you need to register as an Windows insider first with your Microsoft account, before being able to login. If you have already done so for your Windows 10 Desktop, then you are ready to go already.
If you try now, you will be welcomed with a message of “We were not able to find a preview build” and a bit scary looking message about certain “system requirements”, though if Windows 10 Desktop system requirements are some indication, it should not be an issue at all and all Windows Phone 8 / 8.1 devices should be supported, though you may require to be on Windows Phone 8.1’s one of the latest Builds. But with Microsoft you never know and they can always serve a shocker.
When to expect first Windows 10 for Phones Build:
Our sources told us (on January 21) that it may be matter of 2-4 weeks, before we should really expect first Windows 10 preview for our devices. So, we are in fact entering 2nd week already and this app update may hint a sooner release.