Any file deployed to App Engine cannot be altered afterwards. App Engine will copy your deployed files to every instance is starts up. Therefore logging into one instance and editing a file will not alter any other instance's files.
If you require the need to make changes to files after deployment, it is recommended to redeploy your application. If you instead have a static resource that requires the need to be changed live in production, you should host the file off of App Engine such as on
Google Cloud Storage, and simply have your
App Engine code fetch the file whenever it needs to use it.