There are two backups shown in the
~/Library/Application Support/MobileSync/Backup in my laptop.
I want to restore the older backup from it using iTunes.
How can I select one of the backups and ask iTunes to restore the selected backup to iPhone?
You should be able to right click on the phone in iTunes (it used to be on the sidebar, now I think the icon is on top) and select restore from backup. When you do that, a dialog box pops up with a drop down asking you to select which backup you want to use.