How can I delete some unwanted files from my iCloud to provide enough space for backup

As you'd expect, you have to delete data in iCloud. Remember - iCloud is a syncing service so anything you remove from iCloud will also be removed from all your participating devices. If you want to keep it save it elsewhere and back it up.

Or better still, buy more iCloud Drive space. For a small cost per month you can expand your space to a comfortable amount.

Apple offers several options for managing and reducing used space on iCloud in the support document Manage your iCloud storage. See the linked page or the hyperlinked points for more detail. which in the support page are summarised as:

• Reduce the size of your iCloud Backup
• Delete photos in iCloud Photos
• Delete folders or files in iCloud Drive
• Delete texts and attachments in Messages
• Delete messages and manage Mail
• Delete voice memos

Before you delete anything, you might want to archive or make copies of the information that you store in iCloud.