You should use a source repository for all your work - there are free ones out there so there is no excuse
To get the report, browse to the reports folder on the report server, i.e. http://your_report_server/reports/your_folder. If you are permitted to upload reports you will see a button in the bar above your reports that says "Upload file" - that explains one of the ways to upload a report through IE. To get the report, click the Show Details button, click the Edit icon next to the report you want, then click the Edit link on the following screen, you should then be presented with the file download dialog which asks you where to save the RDL report file.
As for security permisssions - it depends, each role can be assigned different levels of access. Anything with Manage reports permission will do the trick, Publisher or Content Manager roles will probably suit.