The quickest way to recover from a expired vCenter root password in a VCF environment is too connect to the vcenter with SSH and it will prompt you to enter a new password. It might be that the SSH session will log you out very fast so have old and new passwords ready! If you do get logged out just log back in and it can also happen that it will not let you log in at all, at that point open a console session and log in there.
As soon as you have put in the new password you can head to SDDC Manager and find the root entry in the security section and choose “REMEDIATE”. This will let you put in the new password you have set in the vCenter. This is all you need to do!
If you have tried to update the vcenter password from the SDDC Manager and it throws you an error it will also hint what you need to do! make sure you cancel so you can use the remediate option.