The update appears as version number 14.6.A.0.368 for the Z1 Compact and Z Ultra. If this is the same as previous Android 5.1 Lollipop updates for the wider Xperia Z range, the patch will include some new customization options, camera improvements, SmartWear integration, and enhanced enterprise support, along with the small number of changes, fixes and improvements included with Android 5.1.
After updating its more recent flagships, Sony has stated that its M2 and M2 Aqua are next in line for a similar firmware upgrade, although a time frame has not been given.
The update is only live in a few regions at the moment, but should be landing in a range of countries over the coming days. Keep an eye out for that OTA update notification or check out Sony’s PC Companion software to see if a manual download is available.