How to recover forgotten root Password in RHEL

You have to login in single-user mode and create a new root password. To enter single-user mode, reboot your computer. If you use the default boot loader, GRUB, you can enter single user mode by performing the following:

Stop 01: At the boot loader menu, use the arrow keys to highlight the installation you want to edit and type [A] to enter into append mode.

Stop 02: You are presented with a prompt that looks similar to the following:

grub append> ro root=LABEL=/

Step 03: Press the Space bar once to add a blank space, then add the word single to tell GRUB to boot into single-user Linux mode. The result should look like the following:

ro root=LABEL=/ single

Step04: Press [Enter] and GRUB will boot single-user Linux mode. After it finishes loading, you will be presented with a shell prompt similar to the following:

sh-2.05b#

Step 05: You can now change the root password by typing

passwd root

Source: RedHat Linux official Side.