If you do not want anything that is currently on the TC it is easier to erase it..
Open the airport utility. Go to the disk tab and select erase.
Data is the root directory of the TC. By Default, although you can name it anything you like.
A quick erase is a couple of min.. the next level erase.. will take many hours.. up to a day.. and the 7 pass and 35 pass will take about that many days.. never ever do it. Quick erase is fine when you are not selling the TC.
If there is data on the TC you want to keep just erase the old backup..
See Q12 from Pondini here.