how do i delete backup from my computer

Just delete the file from one of the following locations

Where iTunes backups are stored on your computer

The folder where your backup data are stored varies depending on the computer's operating system. Make sure the backup folder is included in your periodic data-backup routine.

iTunes places the backup files in the following places:

• Mac: ~/Library/Application Support/MobileSync/Backup/
• Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
  Note: To quickly access the Application Data folder, click Start, and choose Run. Type %appdata% and click OK.
• Windows Vista and Windows 7: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
  Note: To quickly access the AppData folder, click Start. In the search bar, type %appdata% and press the Return key.

(from iTunes: About iOS backups)

Ignore whatheck's post, as that is not the proper way to delete an iPhone backup. iTunes>Preferences(under the edit menu if using Windows)>Devices...highlight the backup listed, hit delete.

bucket118 wrote:

how do i cancel backup of my computer?

You might want to clarify what you're asking ... your thread topic indicates you want to delete a backup, but your post above indicates you want to cancel a backup (which is a different thing entirely).

In windows all my Devices menu options are grayed out -- disabled. I have no idea what to do.