Q: How to install ios8 in 4s

Connect the phone to your computer's iTunes and copy your iTunes purchases over to your computer's iTunes library via File > Devices > Transfer Purchases

Force a backup : File > Devices > Backup

Preferably copy your photos off so that you have your own copy of them : iOS: Import personal photos and videos from iOS devices to your computer

And take your own copy of any important documents, files etc e.g. copy them off via your wifi network, file sharing, email them to yourself, copy them to Dropbox (depending upon what the app supports) - they should be included in the backup but it's best to have your own copy as well

Start the update via Settings > General > Software Update - if you don't have enough free space then you will need to delete items from your phone (the amount of space that each app is taking up is shown in Settings > General > Usage. You will need a lot of free space to be able to download and install the update.

Hi King_Penguin,

I did what you said. However, it still doesn't upgrade. it keeps saying "An unknown error occured (3194). I was wondering if you can tell me what I can do.

Thank you,

Rustam

There is some troubleshooting for that error code on this page : Error 3194, Error 17, or "This device isn't eligible for the requested build"