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Please see the following article from Apple: https://support.apple.com/en-us/HT204904
Upon restart, enter all the registration information and follow the prompts. When you are asked to create your account, enter a User Name and Password. Don't forget this password; you will need it if you ever want to perform additional installations and system updates. You may find it advantageous to include a hint should you forget the password.
Select the appropriate time zone, date, and time. The installation of the system software is now complete. When the computer starts up with the new operating system, you may be prompted to install software updates. Run software update, then restart your computer. Check for additional software updates, install, then restart your computer. Continue checking and installing until there are no software updates available. After updating system software, any application software and the necessary updates can now be installed.