Click on your Apple ID and choose the iCloud option from the side menu. The iCloud storage will be in front of you. Click on Manage to delete old backups.
Select one of the device backups, then click the remove button to delete. If you want to turn off Backup and remove all backups for that device ... Reduce the size of your iCloud... · Delete photos in iCloud Photos
Open your iPhone settings. Tap on your name> icloud> manage icloud storage> icloud backups. Select the backup you want to remove and tap delete.