Launch the Flash Tool software on your computer. For Sony Flash Tool, you can find it in the (Windows) or Applications folder (macOS).
If your phone is locked but ADB debugging was previously enabled, try adb shell rm /data/system/locksettings.db . If not, accept that unlocking via FTF will wipe all user data (photos, contacts, apps). sony c6802 lock remove ftf
Jumping straight to the flash without preparation will brick your C6802. Follow this checklist strictly. Launch the Flash Tool software on your computer