Say a simple shell script, which change some config files.
You can run a simple shell script with a simple task.
Once you create a shell script that does what you want you just need to find an image to run it in and make sure any env vars you need are provided as params to the task.
As for the contents of your script, that isn’t really a Concourse problem but I suspect some kind of
bosh ssh -c ... is what you’re looking for.