hello, i need urgent help about bootloader

i wanted to help my teacher downgrade their tablet android from 15 to 14.... now im shaking and in tears. the bootloader gives oem lock error, and when i tried to leave, now it gives "an error has occured when updating device software", i can't give it back like this... i don't even wanna downgrade now, just wanna go back to factory setting, please someone help, i beg...