I’ve tried to install update 15.7.8 but i fail, What’s wrong?

Hello MohamedAlaa493!

If your iPhone won’t update, please see: If your iPhone or iPad won't update - Apple Support. 

👉 Updating iOS: common issues and solutions - Apple Community

👉 If you see error 9, 4005, 4013, or 4014 when you restore your iPhone

Have a nice day, and good luck!

MohamedAlaa493 Said:

"i’ve tried to install update 15.7.8 but with no solution, What’s wrong? iphone 6s plus"

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Updating an iPhone 6s Plus:

Update Using a Computer:

You can use a Computer to update. Read My User Tip: Making Use of a Computer for Updating an iOS Device - User Tip. If any updates are available, they will install, sure enough.