WD MyBook 3.0 cannot do backup-not enough storage space left

I have a WD MyBook 3.0 3 T.B. external drive. I have set up automatic back ups-now it will not do backups-message-not enough storage space left. I do not want to wipe the drive-how can I free up space (for example remove some older backups I no longer need)

Hello,

What software are you using to backup your computer on this drive?