In our case, we have expired root password for vCenter and we want to reset it !
(we are using vCenter version 7.0.1)
How can we proceed ?
First of all, we have to connect to vCenter on ssh or console :

As you can see, we can’t connect with root password :

So we have to enter command sudo su to connect with sudo user “root”

now as we are logged with super user, we can enter passwd command to initialize password :

You can test it, it works !