In many project where I use SSRS, I create three folders for Development, Acceptance and Production environments. By default, Microsoft does not provides a solution to download and update all reports in one step.
So, I created a open source tool (in C#) that allows to connect on a SSRS server and retrieve all files in your PC (reports, DataSet, DataConnection, images, etc.). You can easily modify these XML files. And finally, this tool allows to transfer all of these files on the server or another SSRS server.
Source code and executable: https://github.com/dvoituron/SsrsReportTransfert