Can you overwrite a recordable CD?

Technically, no - it needs to be a rewritable (RW) rather than recordable (R) disk. However, if you want to erase files because you want to get rid of the disk and have it recycled rather than destroying it, it may be possible to overwrite the files that already on there with something innocuous (like a PDF file of a book), provided the disk hasn't been closed after writing.