What do passwords, in-app data, emails, videos, photos, and information-sensitive memos have in common? Believe it or not, they’re all stored by everyday iPhone or iPad users in their device. What’s worse, is these users are handing over the potential identity theft data to strangers or even after selling the iPhone or iPad without resetting it. So if you’re planning on selling your iOS device, it’s a damn good idea to get rid of all personal info before handing it over. Luckily, removing all data from your iPhone and iPad can be done in a few clicks by performing a factory reset and we’re gonna show you how.

Source : http://www.xtremetechhub.com/2016/06/how-to-reset-iphone-ipad-to-factory.html?m=1

Reset Your iPhone or iPad to Factory Settings
A factory reset takes your device back to its factory settings by resetting the entire system. All the data and apps get erased because the drive in the device gets reformatted. To reset settings of your iPhone (the steps for an iPad are pretty much the same) and get rid of all the data from it, follow these steps:

1. Go to Settings and scroll down until you find a “Reset” tab. You will be presented with a few options of what you can reset, such as keyboard dictionary or home screen layout.
2. To bring your iPhone back to its factory default settings, tap on “Erase All Content and Settings” and then on “Erase iPhone”. If a Passcode or a Restrictions Passcode is set, you will be asked to enter it before being able to reset your device.
3. After you enter the Passcode you will be asked for your Apple ID password. Enter it to begin erasing all data from your device.
4. The reset back to factory settings will run and shouldn’t take more than a few minutes. When you see the welcome screen, it means the factory reset is finished and no personal data is left on the device.
Perform Reset with no Passcode using iTunes
There’s an alternative method to the one above, which can be done using iTunes. It’s great for times when you can’t turn your iPhone on because it got disabled (too many wrong password attempts) or when you don’t remember the passcode. Here’s the steps:

1. Open iTunes. To prevent any potential obstacles during reset, make sure that your iTunes version is up to date – simply click on the ‘check for updates’ in the drop-down menu.
2. Connect your iPhone to your computer. If yo’d like to keep some files, you can do a backup by clicking “Back Up Now”. If you just want to delete everything without saving it first, you can skip the backup.
3. Click on the “Restore iPhone” option in iTunes and confirm your decision by clicking “Agree”. If you ever change your mind and decide that you would like to get the data back on your phone, click “Restore from Backup…”.

For simply reset iPhone, you can try the following solutions:
1. Go to Settings on iPhone, find Reset and choose the one you want;
2. Connect iPhone to iTunes or Finder on Mac, click on "Restore iPhone";
3. Sign in iCloud online and click on "Erase iPhone".
All these three methods can be used to reset your device.

However, for some special situations such as iPhone locked, iPhone disabled, passcode forgotten, Apple ID locked, etc. You will need a professional tool that specializes in unlocking. In such cases, Joyoshare iPasscode Unlocker can be considered. It works well for iPhone, iPad and iPod. Also, you can get solutions to unlock screen lock, Apple ID, MDM, and Screen Time.