If you want to start over completely with Time Machine, you can erase the backup disk to delete all data stored. If you just want to delete some old copies of files, you can open Finder and choose your Time Machine backup disk. Then from a folder named Backups.backupdb, choose the sub-folder with your Mac’s name, scroll up and down to find the folder for the data you want to delete. Right-click the folder, then select Move to Trash.