How can I change the email header template
{{template config_path="design/email/header_template"}}
or footer template
{{template config_path="design/email/footer_template"}}
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Sign up to join this communityYou can override these templates in your custom theme and edit the html as per your requirement.
app/design/frontend/Vendor/themename/Magento_Email/email/header.html
app/design/frontend/Vendor/themename/Magento_Email/email/footer.html
You can override your core header and footer template into your theme folder,
vendor/magento/module-email/view/frontend/email/header.html
To your theme,
app/design/frontend/{Vendorname}/{thememane}/Magento_Email/email/header.html
Same Process for Footer.html file.
app/design/frontend/{Vendorname}/{thememane}/Magento_Email/email/footer.html
Run
php magento setup:upgrade
php magento setup:static-content:deploy
Clear cache.
This setting available in:
Content -> Configuration -> Default Store View -> Transactional Emails
In Magento 2.3.5 you can set it here: Content -> Design -> Configuration --> /YOUR STORE/ --> Transactional emails