Step 1: Prepare your iPhone for restoration
If you have iOS 11.2 or later, go into your iPhone Settings menu — Settings > [Your Name] > iCloud, or go to Settings > iCloud for an older version of iOS — and turn on the items you’d like to back up. We recommend you choose to back up almost everything, but especially your Photos, Mail, Contacts, and Notes. Once you have chosen what you would like to back up (and keep in mind that anything you do not back up will be lost forever once you perform the reset), scroll down until you find the Backup button with the green icon. Tap it and turn on iCloud Backup with the toggle switch. Then, if you’re connected to Wi-Fi, you can tap Back Up Now. Keep in mind that it will take a few minutes.
Alternatively, you can use your PC or laptop. Connect your iPhone to your computer via USB and click the gray Back Up Now button when viewing the Summary tab for your given device in iTunes.
Step 2: Access the Reset menu
Tap the main Settings icon when viewing the home screen, select General from the resulting menu, and tap the Reset button at the bottom of the page.
Step 3: Reset your iPhone
Tap the blue Erase All Content and Settings option near the top, followed by the red Erase iPhone option in the resulting pop-up window to confirm your decision.
Step 4: Restore your iPhone
Once the reset process is complete — a process that may take up to several minutes — check to ensure your phone has been restored to its original factory settings. If done correctly, you’ll once again be presented with the iOS Setup Assistant upon startup.
WHAT DO ALL THOSE DIFFERENT RESET OPTIONS MEAN?
When you’re resetting your iPhone, iOS presents you with a host of different options, and it can be confusing deciding which ones to choose. Usually, you’ll want to perform the aforementioned factory reset, but here’s an explanation of what the other options do so that you can choose the best reset for your needs.
Note: If you are selling your phone or turning it over to someone else, the only one that safely erases everything and takes your phone back to its original state is Erase All Contents and Settings.
Reset All Settings
If you choose this option, you won’t lose all your data, but you will lose your settings and Wi-Fi passwords. Manual settings will disappear, sure, but you’ll still have your pictures, apps, and other content.
Erase All Content and Settings
This is the option we described at the start of this post, which erases everything from your device. Your settings, pictures, apps, and other data will vanish for good — unless you’ve backed up your content.
Reset Network Settings
Sometimes you just need to refresh your network settings to set up a better connection. Choosing this option resets your network settings, flushes the cache, and deletes any temporary files you have stored on your device. It’s great for those who are experiencing trouble with various Wi-Fi connections.
Reset Keyboard Dictionary
Sometimes autocorrect can do more damage than good. If you feel like your iPhone is always suggesting typos and the wrong words to you based on your previous messages, you can always reset the keyboard dictionary.
Reset Home Screen Layout
Although most people like to set up their smartphone’s home screen with their favorite apps, sometimes it’s nice to change things up a bit. If you ever want to go back to the original home screen layout that came with your phone, you can choose this reset option.
Reset Location and Privacy
Sometimes, when you start tapping around in settings, you can mess things up. If you want to go back to your phone’s original privacy and location settings, this is the reset for you. All other info will remain intact.
Source : https://www.digitaltrends.com/mobile/how-to-factory-reset-an-iphone/2/