Installing the latest upgrade of iPhone using iOS 11 ipsw(raw iOS software for iPad, iPhone, and iPod touch, which is used in iTunes to install iOS firmware, iTunes use the IPSW file to update or downgrade iOS version on compatible device) is the simplest and the best method of updating iOS on any Apple device.
This method is simple because, firstly, you can easily downgrade anytime. And secondly, the time consumed is less as the firmware is downloaded on your laptop/Mac without hampering your task on iPhone/iPad if you have space issues. And thirdly you can update iOS in iPhone without wifi.
Install ipsw Manually
You require the correct iOS 11 ipsw files for this. Get the ipsw file for your Apple device and then you have 3 methods to get started.
Before update make sure you have the correct ipsw file and you have backed up all your data. Links for Method 2 & 3 are provided at the End of the post.
STEP 0: Data Backup
This is the most important step for all methods. We need to ensure that no important contact or text is lost after the update.
You can backup everything using iCloud or iTunes. I would recommend to backup using iTunes as the same will not require internet connectivity during the backup and restore process.
Backup using iCloud
Go to Settings > iCloud > Tap Backup
Turn on this to enable backup in iCloud. But note that this will disable backup in iTunes.
Backup Using iTunes-Recommended
Firstly ensure you have latest iTunes installed on your PC/Mac. Once installed, launch it, connect your iOS device to your computer via Lightning cable, select your device from iTunes, then tap on select Backup > To This Computer. This may take a while depending on the number of files you have on your device.
Do not check the Encrypt iPhone backup option. As this will take further time for encryption. Post this step, you are set to fly.
Install ipsw using iTunes
0: Backup of all data (Already done above)
Step 1: Connect the update compatible device to your Mac/PC to iTunes.
Step 2: Once your device is connected to iTunes, the highlighted icon will be activated besides Menu. Click on the icon to go to device information screen.
Step 3: On Summary Tab using Shift (on Windows) or Options (in Mac) key click on Restore iPhone/iPad/iPod button.
Step 4: Browse the downloaded iOS 11 ipsw file. Make sure, after this step your Laptop has ample battery to avoid any power related issues during the upgrade process.
Step 5: Click on Restore.
If you select the wrong IPSW file then iTunes will give an error and update will abort. See the below image.
Step 6: After a few minutes your device will restart with the latest iOS update installed on your device. Recover all data from the backup created in step 0.
And you are done. You have successfully updated your iOS.
For Method 2 & 3 Check below link:
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