don`t boot up and opeen fastboot!

The easiest way I have found for accessing the bootloader / fastboot on most Android phones is to install ADB (a part of Android Studio) and enable Debugging on the phone. With debugging enabled and ADB installed plug your phone into your computer, open a terminal / command prompt window and type "adb devices", without quotes, and hit enter.

If it lists your phone then we can proceed with "adb reboot bootloader", also without quotes.

If you phone is not listed when you use "adb devices" then you will want to make sure the appropriate drivers are installed.