1. Restart your vCenter appliance and wait for the Photon OS Splash screen during boot.
2. Enter e to go to GNU GRUB boot menu editor:
3. Next is to add the following string behind the line that starts with linux: rw init=/bin/bash
4. Enter F10 to boot the changed entry and restart.
5. Enter commnad passwd to create new password:
6. Enter command reboot -f to restart server:
Finally, your password has been changed.