Do you pay for extra storage? If not.. and you are using iCloud to do iPhone backups.. it doesn't take much at all to fill up the free 5 GB. You should either buy more storage, or stop backing up to iCloud and back up to a computer.
You cannot access the contents of an iPhone backup.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Backup methods for iPhone, iPad, and iPod touch - Apple Support
If you are not referring to an iPhone backup to iCloud..
(check Settings > [your name] > iCloud > iCloud Backup)
and you're simply saying that your iCloud storage is full, then you need to manage that storage..
How you do that likely depends quite a bit on what is taking up all your iCloud storage space.
Manage your iCloud storage - Apple Support