How do I instruct iTunes to restore the entire backup files on my iPhone?

Hi,

I had an iPhone 6 and recently purchased an iPhone X. Therefore I backed up my iPhone 6 on iTunes and restored the entire data on my iPhone X. I reset and gave my iPhone 6 to my wife as a spare mobile for which I made a backup again on iTunes, not realizing that it would replace the old data. I had some issues with my iPhone X so I decided to reset it completely since I had it in my mind that I had a backup of my iPhone 6. When i tried restoring it now, it naturally just restores the new data which was replaced. I checked up the backup files and found out that all my files are present there but iTunes only restores the files which were modified. How can I instruct iTunes to restore the entire backup files on my iPhone? Kindly help.