Note: If you are a Linux or OS X user, your device's Android version is lower than 34.3.A.0.228 and you have already unlocked your device's bootloader, then you will need to use Windows for one step of the flashing procedure.
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You need to make sure your Xperia™ is running the latest version of Sony's Android, as a recent change was made to allow the installation of Sailfish X. If your device still has its bootloader locked, then you should be able to update to the latest version of Sony's Android using the built-in Over-The-Air (OTA) update mechanism. To get to the latest version you might need to perform several upgrades.
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Enter your e-mail address, accept the terms and conditions, then submit the form. You will receive an email from Sony giving you a link to click in order to access the next stage... Open the link in the received e-mail and enter the IMEI code of your device (18 digits)... Now read and follow Sony's instructions...