how can I restore backup without erasing the existing data on my iphone?

You cannot do so unfortunately. Any restore of a backup from iCloud or iTunes (finder with MacOS Catalina or Big Sur) is an all or nothing affair. The restore of a backup completely replaces all user data, other than data sync’d into iCloud which re-sync’s to the device (e.g. photos in iCloud, contacts, notes etc, none of which are part of the backup since they are in iCloud).