From the article: https://support.google.com/mail/answer/6567?hl=en
Here's how to change your Gmail password:
- Click the gear icon in the upper right, then select Settings. If you're on a mobile device, switch to desktop view for the full list of settings.
- Open the Accounts and Import tab.
- In the 'Change account settings' section, click Change password.
- In the new window, enter your current password and your new password.