If the address is not hardcoded into a custom template, you can update your address by heading to the Settings page. Click on the tools icon in the upper right corner and select Settings and Billing. Under the account settings section, click on the Sender Info tab and update your address there. That will update the address that displays in most templates.
If you have a custom template and the address is hardcoded into the template, if you can let me know the name of the template and the address that you need the template updated to, I am happy to get that done for you. If that is the case, we can make this post private so only you and myself are able to see it.