Connect it to iTunes and in the sumarry tab, select restore. Note that you can backup your files using iTunes before restoring the iOS. After the iOS restore is done, just restore your backup. And if you are on a lower version of iOS at the moment, you might also consider saving shsh of your device using tinyUmbrella, this ensure that you will be able to downgrade the iOS if there are thing you don't like after iTunes restore your iOS to the latest available one.