You should definitely read about Report Snapshots in MSDN to ensure you have all the necessary background information on report snapshots before you try to dive too deep. Once you’ve gone through that, you can find instructions on accessing report snapshots and snapshot histories here:

• Finding and Viewing Reports in Report Manager
  • To select a report, search for a report by name or description, or browse report server folders to find the report you want. In Reporting Services, published reports and folders are represented as a hierarchy similar to those of file systems and Web sites. To find the reports that you want to run, you can browse through the folder hierarchy. Not all reports that are visible in the folder hierarchy are accessible. Some reports may prompt you for your user name and password to determine whether you can access the data source for the report.
• How to: View and Configure Report History (Management Studio)
  • Start Management Studio and connect to the report server.
  • In Object Explorer, navigate to a report. Reports are located under Home, or in a folder under Home.
  • Expand the report folder.
  • Expand the History folder to view report history.
  • To view an individual snapshot, right-click the snapshot you want to view, and then click View Snapshot. A Web browser opens and displays the snapshot.

Hope that helps.