如何为heroku应用程序更改DATABASE_URL ?

I've an external database ready and want to now use that database with my heroku app. But I'm unable to edit the Configuration Variables. I tried using GUI, Which says, Cannot overwrite attachment values DATABASE_URL. While I tried using CLI as well. I used the command: heroku config:addDATABASE_URL="postgresql://username:password@IP:PORT". However, this throws an error ........ is not a heroku command.I've an external database ready and want to now