If you are looking for a way to backup .pbix files from www.powerbi.com, you are at the right place.
It could be difficult to keep trace of changes made on PowerBI Service. The azure function project available here allow you to copy pbix from the Service to Azure Blob Storage.
To run project localy, add a local.settings.json file to the project. You can find a sample file "PowerBIServiceBackup/sample_local.settings.json".