ReportServer keeps a copy of the last uploaded reports files - up to six generations deep (backups 0 to 5).

 

Loading a backup file is easy, but should only be done if you think you have lost the original reports file.

 

Go to "File > Open backup > From server".  The menu item will expand once again to show 'Backup 0 (latest)' though 'Backup 5 (oldest)', just pick the backup you would like to download.