The only reason you would want to keep old backups is if you still needed to restore some of the data from them to your device. It will only delete it from iCloud, freeing up space in your account. It won't do anything to the data on the data on your phone.

Is it OK to delete old iPhone backups?

A: The short answer is no—deleting your old iPhone backup from iCloud is completely safe and won't affect any of the data on your actual iPhone. In fact, even deleting the backup of your current iPhone won't have any impact on what's actually on your device.

What happens if I delete my backups on iPhone?

Deleting a backup deletes the backup only from the iCloud storage, not anything on the iPhone. The iPhone's iCloud backup will be updated at some point - automatically when the iPhone is connected to an available wi-fi network while connected to a power source unless the iCloud backup is turned off on the iPhone.