If you are using your own API, you can definitely do this. If you are using our Constant Contact sign up form generator, we do not have the functionality in this program for updating due to security reasons. If a person knows a contact email address they can just update it without verifying who they are. However, we do have an update version for people who understand this risk.
What are you using? A custom API, or something that we have created here? I can definitely help you either way!
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