There's no official changelog yet, we may only ever get one as part of the production release in early February, but I'll update this if one such appears.
Anyway, better to be safe than sorry, so definitely update and, as usual, there might be a security implication that's under the hood. Watch this space for more news of the various Insiders rings over the next few weeks.
Anyone with a smartphone running the 'Release Preview' Insiders ring of Windows 10 Mobile 'Anniversary Update' will get this update over-the-air soon. Your phone will tell you within a few hours, or you can check manually in Settings/Updates & Security/Phone update. Stay patched and up to date out there!