Long-term Sony Xperia users will be very familiar with Flashtool, it has been the da facto method of flashing firmware of Xperia devices to a different region. It has been around since the advent of Sony’s Android Xperia phones, but support was not added for any of the 2017 Xperia devices.
However, Flashtool developer Androxyde has updated the application to version 0.9.24.2, which now includes support for the Xperia XZ Premium, Xperia XZ1 and Xperia XZ1 Compact. Do note though that this new version of Flashtool has moved from Java 8 to Java 9, so you will need to run a 64-bit operating system. If you want to download the new version, click here.