how can back up if my phone es dead

If the phone is dead, it is too late to back it up. You need to make an appointment at the nearest Apple Store for an evaluation and explanation of your options.

Apple Store - Find a Store - Apple

It is impossible to backup any computing device that won’t power on and function normally. That’s why backups must be made and kept up to date before something happens. After it already has happened, you simply lose your data.