You can remove the old backup, however how are you going to be backing up your iPhone so you can restore your data if you experience a problem? You also have the option to increase your iCloud storage to make sure you have the ability to backup your device daily. See this, Upgrade to iCloud+ - Apple Support
But to answer your direct question, how to delete a backup in iCloud, on your iPhone open Settings><your name>iCloud. You will see a graphic of your iCloud storage and directly beneath that is Manage Storage. Tap on that, on the next page you will find Backups. It will show you what backups you have. Tap on the backup in question, then tap Delete Backup if you wish to delete it.