i couldnt find the software update above then 15.8 version on my iphone 6s plus

That is correct.

An iPhone receiving a restored backup must have an iOS version greater than or equal to the iOS version that created the backup.

The iPhone 6S+, released by Apple over eight years ago, is not capable of receiving a backup from an iOS 17 or iOS 16 iPhone. The iPhone 6S+ cannot use an iOS version greater than 15.8.

That's correct. You cannot download a backup made on a higher level iOS version to an iPhone which cannot run at least the same iOS level or higher.

You can set up the iPhone manually, by signing into your Apple ID on the iPhone and download your apps, setup you mail account and sync whatever data you synced to iCloud back to the phone.

The iPhone 6s Plus cannot be updated beyond iOS 15.8. It lacks the components needed to run newer software.