Deleting stored files to eliminate need to purchase more space

As part of my Norton 360 software is has automatic backup.  I honestly have no idea what it is backing up but for the first time since I got it several years ago, it is telling me I am running out of storage space an need to buy more.  Since I also have Carbonite running I doubt I really need to worry about Norton backup but thought it was nice to have in case anything ever goes wrong with Carbonite.  I don't want to buy more space.  How do I find out what is being stored and how do I reduce the number of files stored there that probably aren't really that necessary?

Requested move to Backup Forum.