How to delete backups shown in about my computer
If Time Machine is enabled, this is (probably) a Local Snapshot. These allow Time Machine to back up your files even when a backup disk is not available. However, OS X automatically deletes these as space is used up. From the Apple Knowledge Base:

To make sure that you have storage space when you need it, Time Machine creates and retains local snapshots only when there is plenty of free space on your startup drive:

If less than 20% of the total storage space on your startup drive is available, Time Machine removes local snapshots, starting with the oldest, until you have more than 20% free space.

If less than 10% or less than 5GB of storage space is available, Time Machine removes local snapshots more quickly. When only one snapshot remains, Time Machine stops creating new snapshots. As free space increases, Time Machine at first replaces the previous snapshot with a new one, then eventually resumes creating snapshots as normal.



If you need to free up space used by this remaining snapshot, disable Time Machine and it will be deleted. However, this means you won't have backups, which is not a good idea. If you want to disable local snapshots, run this Terminal command:
sudo tmutil disablelocal

If this doesn't delete the local snapshot, disable Time Machine until it is removed and enable it again.
For more information/troubleshooting regarding disabling Local Snapshots, read this forum discussion.