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Choosing a secure port - 443 does not enable the https to backend APIs.
It should be enabled by posting data with parameter SSLInfo using management APIs.
It can not be updated from Apigee Edge UI portal.
Following are the details for enabling the existing target server with https.
curl -X PUT --header "Content-Type: application/json" --header "Authorization: <Basic Auth Value>" -d "{ \"name\" : \"<target_server_name>\", \"host\" : \"<host_name>\", \"isEnabled\" : true, \"port\" :<port>, \"sSLInfo\": { \"enabled\": \"true\" } }" https://api.enterprise.apigee.com/v1/organizations/<org_name>/environments/<env_name>; /targetservers/<target_server_name>