Here are the steps:

• Enable developer options
• Enable USB Debugging
• Download adb and fastboot. Example below on debian/Ubuntu

sudo apt install adb fastboot

• Go the the following page: developer.sony.com/open-source/aosp-on-xperia-open-devices/get-started/unlock-bootloader/how-to-unlock-bootloader and get your unlock code. Follow the steps outlined there

Source(s):

1. developer.sony.com/open-source/aosp-on-xperia-open-devices/get-started/unlock-bootloader/how-to-unlock-bootloader