If you need to create a new backup from Mac
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
On your Mac, click Back Up Later.
Click the Time Machine icon in the menu bar, then choose Enter Time Machine.
If the Time Machine icon isn’t in the menu bar, choose Apple menu > System Preferences, click Time Machine, then select “Show Time Machine in menu bar.”
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.
To minimize the risk of needing to create a new backup, consider doing the following:
Ensure that your external backup disk’s firmware is up to date. If you back up to a Time Capsule, check the Apple Support website to be sure you have the most current firmware available.
If given the choice between AFP and SMB, use SMB to back up to your external backup disk.
Use an uninterruptible power supply (UPS) with your external backup disk to prevent unexpected power failures.
If allowed, disable write caching on your network storage device.