When the pop-up message appears, click "Erase" to confirm it. When the resetting is complete, there will be the "Slide to set up" message on your i Phone's screen. If your i Phone has been unlocked before, connecting it to i Tunes will reapply the unlock code, allowing you to use the phone on other networks again. After the unlock message, you will be prompted to restore your i Phone.
When you receive a message saying "An update to the carrier settings on your i Phone is available. Select the backup that you just created and then wait until the restore is complete.
In this way, some settings will also be backed up and restored to your i Phone later.
If this is not what you want, you can choose to backup your i Phone to your computer selectively according to your need by using i Skysoft i Phone Data Recovery or i Skysoft i Phone Data Recovery for Mac.
This way is the best way to restore your i Phone without updating i OS, and it's available for every i Phone user. You can choose to use i Tunes or i Cloud to get them backed up.
If you have important data on your i Phone, you're suggested to back them up first.
During this time, please click "Cancel" when you're asked if you want to upgraded to a newer version of i OS.