Although there's a changelog, it's for the wider desktop/tablet arena, it's not clear what's changed for mobile, but expect a few under the good bug-fixes and reported battery optimisations.
If you're confused by the 'branch' terminology, then see my infographic from back in March, which is still relevant, though will need updating later this year when 'Redstone 2' builds come on-stream:
The trick to getting this update, for older 8.1-Insider-updated devices like the 820, 920 and 1020, is to go into the Windows Insider application and use the '...' menu item 'Leave the programme'. Once off the programme, it seems that Microsoft's servers do still contain provisioning for the older devices (though heaven knows how long for, so be quick) and they then get the production update listed here. Phew!
Also, with a little hacking, you can jump the rings and take some of the older, 'blue' devices above (1GB RAM, and that are already on Threshold) right into 'yellow' (ahem) and beyond, ignoring the issues mentioned above.
Comments welcome if 10586.545 fixes something major for you!
PS. On the timing of the Anniversary Update rollout, i.e. Redstone, for mobile, I did say, several times, that it wasn't ready for public consumption and there's still no sign of it. We might even get another Fast ring Insiders update to test before the mass rollout. So much for 'August 2nd'!