(Ensure you do not share any of your personal information such as serial, phone number, email ID, etc...).
Have you attempted any steps on your own before posting on the HP Forums?
your username is the email address of your Google account and it cannot be changed
Your Chromebook's password is your Google account password.
You can go here, or you can click on your picture in the upper right hand corner of any Google service page, click Account and click on the Security tab.
While you're changing your password, please set up your Recovery and alerts and consider enabling 2-step verification. A little forethought now can save you a huge migraine headache if you ever forget your password or if someone tries to hack your account.
Once you're done changing your password, sign out of your Chromebook and then sign back in with your new credentials.
Please let me know if this resolves the issue, or if you require further assistance!
Eagerly waiting for your response!
I hope you have a good day ahead,
And Feel free to ask your queries as this forum has some of the best people in the world available and ready to help.