How do I change the Salesforce Sync User?

The Salesforce sync user is the person the primary sync runs under. The primary sync is responsible for pulling data from Mailchimp and importing it into Salesforce. You can change this user by logging in as the user you wish to change to. Open the Mailchimp settings tab and select General Settings. Click the icon next to the current user's name on the Salesforce Sync User.  A pop-up will appear that will confirm you are changing the Salesforce Sync User to your user.

Don't forget to change the Connected Mailchimp Account user as well, if needed.