I've just restored my iPhone to default factory settings and I would like to share the steps with you.
1) Open iTunes and connect your iPhone into your Mac
2) Press and hold Sleep/Wake Button and Home Button at the same time
3) Release Sleep/Wake Button as soon as the iPhone screen goes blank. Continue pressing the Home button until iTunes prompt you that it has detected an iPhone in recovery mode.
4) In iTunes, click Restore
5) Then click Restore & Update
6) iTunes will now restore & update your iPhone
7) iTunes will let you know once it's done, just click OK.
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8)You're done! :)
