Well, you can’t 
the only way to change the value of that field is through the email validation feature included in Bubble.
This field is set to “no” by default. If you tick the “Send an email to confirm the email” checkbox in the screenshot below, the user will receive an email from Bubble, with a link redirecting the page you set in “Confirmation page”. When clicked, the field will be set to “yes”.
There are ways to use manual email validation though. If you’re interested in doing that, I would advise to read this: How to custom validate a new user email