Pres the Power button and the Volume Up (in some models you need to pres in one time volume up and down buttons) button and the menu will appear. It’s the Factory Mode screen menu.
You should choose ‘Clear eMMC’ to reset your phone. You can do this by pressing the Volume Up button to navigate the menu. After choosing the ‘Clear eMMC’ option, press the Volume Down button to perform software reset. It will be quick and short erasing the internal memory of your smartphone and will reboot automatically. After the reboot, the annoying ‘locked’ unlock screen will disappear.