If your iDevice starts to act strangely and you have run through the size of common troubleshooting fixes, recovery mode can be your answer. This allows you to easily restore your device and reinstall iOS with iTunes.
When you reinstall iOS, there is a risk that you may lose all data on your phone, so it's good to use normal backups on your computer via iTunes or iCloud. By saying this, you can boot your iOS device in recovery mode.
First of all, make sure you have the latest iTunes version
First, make sure you are using the latest version of iTunes. With the application open, go to iTunes> About iTunes.
Please note which version you use and check it on this official Apple Support page to see if you are on the latest release.
With that out of the way, you're ready to get started. The rest of the procedure varies slightly depending on which device you use, so we go over them one at a time.
For iPhone 7 / iPhone 7 Plus or later
If you're using an iPhone 7, 7 Plus, 8, 8 Plus, X, XS, XS Max or XR, here's what to do.
First, make sure the phone is turned off. Then press and release the volume up button.
Next, press and release the volume button immediately.
Now hold the side button, the only one on the side of the volume buttons. Do not drop the button even if the Apple logo flashes on the screen. Continue to stay until the Reset mode screen appears.
When this screen appears, continue and connect the device to the computer via the flash cable.
For iPhone 6s or earlier and most iPads
These instructions cover the iPhone 6s and earlier models, as well as most other iPad models than iPad Pro 11 and 12.9-inch. For the two, check the next section.
First, make sure the device is turned off. Then press and release the volume up button.
Next, press and hold the Home and Sleep / Wake button. The Sleep / Wake button is on the iPhone 6 or later page, and at the top right of the iPhone 5 and earlier. Do not drop the buttons even when the Apple logo flashes on the screen. Continue to stay until the Reset mode screen appears.
When done, proceed and connect the device to your computer.
For iPad Pro 11 inch or iPad Pro 12.9-inch  Press the volume up button fast and quickly, then press and release the volume down button quickly. Press and hold the Sleep / Standby button at the top of the device until it is restarted.
When recovery mode is started, connect the device to the computer.
What do you do when you are restored?  Now that the device is in Recovery mode, you have about 15 minutes before it automatically expires. If you do not move fast enough and your phone goes out of recovery mode, repeat the same button press as explained above to re-enter it.
A window like this will appear on your computer once you have entered Recovery Mode on your phone or tablet. When you see the "Reset or Update" options, select Update.
You will want to try "Refresh" instead of "Reset" because your problems can be solved by simply updating your iPhone, which will keep all your content and personal preferences. If this does not work, you may be able to select the "Reset" option, which deletes all content and personal information on your device. Do not accidentally click "Reset" if you do not have a working backup of your data or want to review that process and make sure you try "Refresh" first to save yourself about potential heartache.
Select "Update," Then, and iTunes will work to reinstall iOS on your phone while leaving your data intact. When the update or recovery is complete, you will be prompted to configure the device.