This is something you should do regularly. Every six months, just check all your passwords and make sure you don’t need to change them. Most of the time you should as a precaution.
To do so in Gmail, head to the settings page and then “accounts and import”. There, at the top of the screen, you should see the option to “change password” and “change password recovery options”.
Remember, don’t choose something obvious. Regularly changing your passwords, and keeping all your passwords different from one another, protects you from hacking attacks.