Your Must install OST LA 6.0.4 on Your PC first!!!
* Your warranty is now void after U unlock Your Bootloader.
* We are not responsible for anything that may happen to your phone by using this method
* You do it at your own risk and take the responsibility upon yourself and you are not to blame us or XDA and its respected developers.
1，Perform a factory reset on your device.
2，U need to find where You install the OST LA files. Usually it is under C:\Program Files (x86)\OST LA
3，Launch PowerShell under this folder
4，Put Your Phone into the Download Mode
5，U need to prepare the *.img You want to flash
6，First Use this command:".\fastboot-android reboot-bootloader"
7，Second:".\fastboot-android flash system F:\Nokia7Plus\PE.img" Remember After the "system" U need to type in where u put Your img files.
8，Third:".\fastboot-android -w"this will wipe all the data in your phone.
9，Finally:".\fastboot reboot"This will reboot ur phone.