Here’s how to restore and then install iOS 9 on your device. It takes more time than just upgrading the already-installed operating system, and that doesn’t include the time it takes to reinstall your apps. This is done by restoring your iOS device using iTunes. If it’s available, the version of iOS 9 for your device will download to your computer for installation.
In the right window, click on the Check for Update button. If you don’t see an alert, click on the device icon in the upper left and then click on the Summary tab in the left column. If you want to download the update but install it later, click Download. Click the Download and Update button to install iOS 9 right away.
The download and installation will take a few minutes.
Anything above 60 percent should be fine, but the weaker your wireless connection, the more taxing the connection will be on your battery.
If you enter your Apple ID and password and still can’t sign in, visit Apple Support: Find My iPhone Activation Lock.