Do, however, make a note that the update is hitting only the single-SIM Xperia X (F5121) variant currently. The dual-SIM variant (F5122) should receive the update very soon.
As far as the size is concerned, the update weighs around 93MB, so it shouldn’t be a problem to download it over cellular data. Just make sure you are in the strong signal zone or alternatively connect to a Wi-Fi network to download the update.
In typical Sony fashion, there’s no changelog included. If we were to guesstimate, it should bring along with it fixes to the bugs existing in the previous build, especially the ones surrounding Bluetooth.
As is the case with any update, make sure to back up your personal data just in case things go wrong.
Source: Xperia Blog