Sony has begun rolling out the Android 7.0 Nougat firmware update to Xperia Z devices that currently include Xperia Z5 and Xperia Z5 Premium, whereas it is expected to reach Xperia Z5 Compact some time later. The Android Nougat update is being pushed via an OTA update for select variants of phone. This means that the Android Nougat rollout is region-specific and you should keep checking your phone for update notifications.
The Android 7.0 Nougat update roadmap laid out by Sony earlier already included Xperia Z5 series smartphones that's happening now. According to Xperia Blog, this update is 1.2GB in size and brings a lot of Android Nougat features like multi-window support, self-timer button for front camera, and Google Now integration in Xperia launcher. Sony has also updated the battery optimisation settings on eligible Xperia Z5 smartphones. The update will get all the eligible Xperia Z5 devices a new build number 32.3.A.0.372. Sony in its update log on Xperia Z5 devices also mentions that you cannot downgrade your device to a lower Android version.
As we mentioned, the Android Nougat rollout is happening for select Xperia Z5 devices, which means that depending on where you stay, the update might not reach as you'd expect. Additionally, if your Xperia Z5 is a carrier-bound device, you may suffer more delays as the update will have to go through the approval process from your respective carrier.
Sony has already updated a number of devices that are a part of its Android Nougat list, namely Xperia X Performance, Xperia XZ, Xperia X, Xperia X Dual, and Xperia X Compact. Notably, Sony in its Android 7.0 Nougat plan, had earmarked more devices, in addition to those already updated, like Xperia Z3+ and Xperia Z4 tablet that should be getting the update around December. The last batch of Xperia smartphones to receive the update will be Xperia XA and Xperia XA Ultra, which are slated to receive Android 7.0 Nougat in early 2017. The Sony Xperia Z4 and Xperia Z4v, released in the second quarter of 2015, have been entirely left out of the Nougat rollout roadmap by the company.