Sony Xperia XZ1 Manual page 11

Your device does not revert to an earlier software version of Android when you
perform a factory data reset.
My device freezes or applications suddenly shut down
Xperia™ XZ1 Dual SIM support overview

Downloaded applications can cause your device to behave unexpectedly or affect its
performance. Use safe mode to check if any downloaded applications are causing the
problem. In safe mode, your device only starts with software and applications that were
already installed when you purchased your device. If the device's performance improves
in safe mode, it's likely that one or of your post-purchase downloaded applications are
affecting the device negatively. You can exit safe mode and restart the device to
uninstall the application or applications that you suspect are causing the problem. If the
problem is recent and you're not sure which application is causing the problem, you can
start by uninstalling the most recently downloaded application.
To enable Safe mode
1. Turn off your device.
2. Press and hold down the power key
3. When the Xperia™ logotype appears, release the power key, then press and hold
the volume down key until Safe mode appears on the screen.
To exit Safe mode, restart the device.
To uninstall an application from the Application screen
Only downloaded apps can be fully uninstalled.
4. Touch and hold the app icon until the device vibrates.
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