1. Login to Gmail or Google (In google home page right top there is a sign in link)
2. Click on the settings link on the top
if you are on gmail
If you are on Google home page
3. Choose accounts and Import and click on Google account settings
4. Click on change password under Personal settings
5. Enter old password and New passwords and save
6. On step 4 Click Change password recovery options
7. Verify your details and change the security question
check this too
Simplest way go to https://www.google.com/accounts/EditPasswd (after login, or it will ask to login) and change password.
(But i prefer you go the above method since it does not need to click on any external site, including this site. Or copy paste the link to avoid phishing)