[SalesForce] How to update a field in target Data Extension in marketing cloud

I wrote a query to update a field inside a target Data Extension. The query will set the field to True when a subscriber has received an email within three days. The field has a default value of False.

Problem: The query is setting the field inside the target DE to false. How can i update the field inside the Target DE. Here is the query below:

Select [Email Address] as [Email Address], contact_three_days as 'True' from initial_emailing_list 

WHERE [Email Address] NOT IN 

(SELECT SubscriberKey FROM _Sent WHERE EventDate >= DATEADD(DAY, -3, GETDATE()))