I tried "ManiIOT"'s solution and it worked surprisingly.
I've added another role (Compute Admin Role) for my google user account from IAM admin. Then stopped and restarted the VM.
Afterwards 'sudo passwd' let me to generate a new password for the user.
So here are steps.
- Go to IAM & Admin
- Select IAM
- Find your user name service account (basically your google account) and click Edit-member
- Add another role --> select 'Compute Engine' - 'Compute Admin'
- Restart your Compute VM
- open SSH shell and run the command 'sudo passwd'
- enter a brand new password. VoilĂ !