There is 2 easy steps to change root password for WHM
Once you’ve logged into WHM with the current root password, select Change Root Password from the list on the left, under Server Configuration. (Home >> Server Configurations >> Change Root Password)
Once you’ve selected a new root password (you can use any combination of upper- and lower-case letters, numbers and special characters to increase the password strength), enter the new password twice, then click the Change Password button.
That’s all there is to it; you now can close out of WHM and log back in with the new password.