You can reset the root password of any (recent) VMWare appliance, such as the vCenter Server Appliance (VCSA), or Platform Services Controller (PSC) by following the procedure outlined here. Note that you will need to have physical or console access to perform the reset. The reset also requires a restart of the appliance so you'll need to schedule downtime for it.
First of, take a snapshot or backup of the virtual appliance before proceeding. In case the reset should fail, you'll always have a backup to go back to.
Restart the appliance. Right after the BIOS screen, the PhotonOS splash screen will appear for a few seconds.
During this time, press e to enter the GNU GRUB edit menu, allowing you to modify the boot parameters of the OS.
With the up/down arrow keys, navigate to the line starting with linux. Use the left/right keys to navigate the cursor all the way to the end of that line, so it's set right after consoleblank=0. Append the following text to the line:
Press F10 to boot the OS. It will take a very short time and when completed, a prompt will show. Note that during this time, none of the services (including networking) will have started.
By default, the root partition will be mounted read-only. Perform a remount to make it writable by running:
mount -o remount,rw /
Next, type to change the password. You will be prompted for a new password twice:
Note that you will not see anything (no *) when typing the password.
When the password has been changed successfully, unmount the root partition. This is important so as to prevent a dirty volume and to make sure the changes is written to disk:
Reboot the appliance and let it start up normally:
After the appliance has started again, and all services have been started (this may take a while), confirm the changed root password by logging on the VAM webpage or SSH. Also, if you've created a snapshot (step 1) of the appliance, you can remove it now.