Ideally the same way we define an EC2 instance, perhaps bound directly to an S3 bucket resource defined in the same script. Ideally reading the config definition from an S3 file that we can update at will.
If the file is PUT in a S3 bucket via SFTP, you can trigger a lambda that run CF template.
But that seems kind of Rube Goldbergish. Why not make the change to the file, push it to Git and use CodePipeline with lambda?