How can I update my iPhone 3gs to ios6 so my nephew can use it as an iPod?
I had a 3GS on iOS 6 for a whole year, until I went to a 5S &amp; iOS 7 when that was released. 

It was a bit slow &amp; clunky compared to a newer device, but it worked just fine for what I needed. My main usage was as an e-reader &amp; iPod, plus some mail &amp; web.

One thing I did find was that after reducing the total memory used on the device [in my case by removing about 3GB of songs from it] to about 10GB total used on a 16GB phone, it appeared to run noticeably faster than when it was near full.

Depending on how you are going to set up your nephew's phone, it's probably best to do this on his computer, setting it up as a new device under his own Apple ID [this will, of course, wipe all data from it. All apps etc would need to be re-purchased under that account] - otherwise he'll need your ID &amp; password &amp; also it's likely you'll be sharing the same diary &amp; contacts etc... not really what you'd want, I presume.
You might want to see if Family Sharing would be a useful option - individual accounts, shared apps &amp; songs, one credit card, parental controls.

Once set up as a new device, it may do the update automatically, as part of the setup process - if not...

You can source the updater file from http://ipswdownloader.com just by dialling in the required iPhone [get the latest available for that device, ie 6.1.6 as Apple will not sign anything older than the latest the device can run]. 

Launch iTunes, with the device connected
Hold the alt/option key whilst clicking "Check for Update". This will bring up a file-picker where you can navigate to the ipsw file &amp; start the update process.