If you see a message that you must create a new backup
If you must create a new backup, that means a problem has occurred that prevents Time Machine from using the existing backup.
Try to retrieve data from the existing backup: Click Back Up Later. Then click the Time Machine icon
in the menu bar, and choose Enter Time Machine. Use the arrows and timeline to browse the backups that contain the files you want to retrieve. Time Machine asks you again later to start a new backup.Create a new backup: Click Start New Backup. Your existing backup is deleted, and your data is fully backed up only after the new backup is completed.
If you back up to a Time Capsule, check the Apple Support website to be sure you have the most current firmware available.
You may get a backup verification failure message if you use network backup solutions from manufacturers other than Apple or you have a hardware problem with the backup disk.