iPod touch 4th generation won't update to ios7

hello I have the iPod touch 4th generation I recently got it back from my little sister in law and when I turned it on it gave me the ios7 update but then I resettled the iPod touch to it's factory settings I guess it lost the update and now won't give it to me keeps telling me it's up to date. I've read the fourth generation doesn't supoort ios7 but I had it already!