This is simplest way to change forgotten CentOS 7 Root Password and this is totally different from CentOS 6
1:- In the boot Grub Menu Press “e” to open edit mode.
2:- When you press “e “this edit window will open and go to the line of linux16
3:- Replace ro with rw init=/sysroot/bin/sh
4:- Now Press CTRL+X to start Single User Mode.
5:- Access system with chroot command and change password with passwd command,Update SElinux and exit from shell and now reboot system.
System take more time than normal boot.