Unlock bootloader.
Install TWRP recovery.
Boot to TWRP (Go to bootloader screen by Power off, followed by Vol – and Power On button pressed simultaneously, then Power button released; Next Vol – to highlight recovery menu, and press Vol +)/
Install Superuser latest package from TWRP.
Reboot, and install SuperSU app from Play Store.
Also install busybox with Busybox installer by sterricson, from the Play Store.