Reboot the vCenter Server Appliance.
After the VCSA Photon OS starts, press the e key to enter the GNU GRUB Edit Menu.
Locate the line that begins with the word Linux.
Append these entries to the end of the line:

rw init=/bin/bash

Press F10 to continue booting.

Run the command
mount -o remount,rw /
In the Command prompt, enter the command passwd and provide a new root password (twice for confirmation):
passwd
Unmount the filesystem by running this command (yes, the unmount command is umount  –  it’s not a spelling error):
umount /
Reboot the vCenter Server Appliance by running this command:
reboot -f