iPhone, iPad, and iPod touch backups are all managed by iTunes automatically, but you can manually delete older backups from your current devices or backups from devices you no longer own.
Launch iTunes, open iTunes Preferences, select the Device tab. It looks something like this usually:
You'll be presented with a list of the backups that currently reside on your computer. If you sync more than one device there will be entries for each device in the list. Note that they are listed in chronological order with the most recent backup being the most important one. So probably don't want to delete that one.
If you find a backup that you need to delete locate it in the list and select it. Click the Delete Backup button and iTunes will delete it and remove it from the list.
Backups that display a small padlock are encrypted.
