If you've written to a CDR then that's it, there's no going back.
If the CD was written as 'multi-session' then you can add more but you can't delete.
Unless you've used a rewritable CD ie a CDRW. If so then you'll be able to erase the disk in Nero. The Erase Rewritable option can be found in the Recorder menu. I'm sure other CD burning software will let you erase CDRWs as well.