After the initial PC Backup is run, all the files that are selected for backup will stay on the server for as long as your PC Backup account is active. Only the files that are deselected from the list of files to backup, or files that you have deleted from your machine, will be deleted from the PC Backup servers after 30 days.
During the scheduled backup, PC Backup checks to see if there were any changes made to the files since the last backup. If there have been changes, the new versions of the files will be uploaded to the server during the next backup. The old versions of the files will still stay on the server for 30 days.