Assuming it's an upgrade and not a downgrade, all you have to do is connect your iPhone by USB to your laptop, open iTunes, then a message should pop up asking you to upgrade your phone. If you don't want to upgrade it to the current version however you may have to try and find an archived version online.
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Assuming it's an upgrade and not a downgrade, all you have to do is connect your iPhone by USB to your laptop, open iTunes, then a message should pop up asking you to upgrade your phone. If you don't want to upgrade it to the current version however you may have to try and find an archived version online.