If you are using any one of the Google Services, than it means you have a GMail account. So that changing GMail password means that you changing your Google account password so that any account related with your GMail account will also have their passwords changed. This guide give you step by step instructions to change your GMail password:
First you need to log in your GMail account by following this link.After you login, you should click on your account name at the top right corner. This will popup a small box. You should click on Account link. This will open google Account Settings.
You should select Security tab, second in the row. From this page clicking on "Change password link will lead you to password change page.
You should fill the password change form.
New Password: Your new password. If you want a bot to create random, easy to remember safe password you can use this site.
Confirm New Password:Same password you entered above. Please do not forget write down your new password a small paper and replace with your old password. Also password change is immediate, so when you set the new password, your old password does not work anymore.