Need info on file backups

Keep getting blaring notices on my "Norton 360"  that my backup is almost full, do I want to "buy" more backup space? Are my backups stacked on each other, or do new backups overwrite the old?

 

Can't really afford $40 for more space, have changed settings from automatic to manual backup. Moreover, I use an external drive to backup my computer every six weeks.

 

Can I erase old backups that are no longer usefull to me or to Norton, and thus avoid having to buy space?

 

Thank you for your assistance!!