There's no indication of a changelog so far but I'll update this post if one emerges. Users on various sites are reporting that some longstanding bugs in the UI (e.g. delays) have gone away, so expect a smoother experience.
Anyone with an existing phone that's officially deemed Windows 10 Mobile compatible by Microsoft can join in the fun. The full list of phones includes Lumia 430, 435, 532, 535, 540, 635 (1GB RAM version), 640, 640 XL, 730/735, 830, 930, 1520, Icon, BLU Win HD w510u, BLU Win HD LTE x150q, and the MCJ Madosma Q501. Plus, of course, all the devices which have shipped with Windows 10 Mobile in the last year, of which the Lumia 550, 650, 950, 950 XL, Acer Jade Primo and HP Elite X3 are the highest profile.
In each case:
- Upgrade to Windows 10 Mobile (if appropriate) using the Upgrade Advisor.
- After at least one update cycle you should then be on the Anniversary Update (a.k.a. Redstone). Head into Settings/Update & Security/Windows Insider Programme
- Join the Insiders programme and choose the 'Release Preview' level in the pick list.
- Within a few hours your phone should be seeing the build mentioned above.