Hi,
You don't really have an Aqua Mail account. You can only add existing mail accounts into the app. To change the password of an existing mail account, you'll have to visit your mail provider's website - i.e. for ikippen@gmail.com you have to change the password at the gmail.com website.
If you're referring to the app lock password, that is setup in Settings > Security > App access control. You cannot change the password but you can turn off (choose None) the app lock and then turn it back on, which prompts you for a new password.
Martin