When a backup is in the progress of running there is an option to stop the backup. This button is situated on the top right above the progress bars on your My PC Backup application.
Stopping a backup does not mean the backup has to start all over again next time, it will pick up exactly where it left off with the file being uploaded at the stop time.
If a backup is stopped you can then choose new files to backup and start the backup again when you wish. If you do not manually start the backup it will automatically run at the scheduled time.