Open vSphere Client and login with the root username and password. Once it opens..
Click the Local Users & Groups tab.
Right click root and select Edit
Click the check box "Change Password"
Type the new password twice below.
**Now I want to note this. I did this the same time on two ESXi servers. One change immeditatily, but the other took a while (more like 1 hr +). After changing the password, I could login to the root account with the old and new passwords. I did nothing more after changing it and I just retried it. The new password is the only password that works now. **
How to change a forgotten root password: Link