External HD.
You cannot alter Time machine backups, but there is now reason to as you can restore from a backup, alter the files on your internal HD, and then make a new time machine backup with the altered information.
Deleting them data off time machine is problematic, and therefore not advised.
Time machine deletes old backups to make way for new when the disk gets near full.
If you want to, the best way to clean up the external HD is to reformat (erase it) using disk utility, and start fresh.