How to restore from a Time Machine backup when there is already a user account set up?
Migration Assistant

This is the built in data transfer tool for Macs.

You can find it in /Applications/Utilities. You'll want to do the following:


Plug in your Time Machine backup drive, open Migration Assistant and select the From another Mac, PC, Time Machine backup, or other disk option:

Select the From a Time Machine backup or other disk option:

Select your Time Machine backup and click Next.
It should show you your old user account, which will have a drop down menu to select all the things you can restore.
Follow the prompts to restore.


This method will create a new user account alongside the one the Apple Store folks created for you. You'll be able to log out of the account they set up and log into your own. 

Another option would be to:


Navigate to /var/db by pressing Shift + &#8984; + G and typing in the path.
Find the .AppleSetupDone file and delete it. It will likely ask you to authenticate as an administrator.
Restart your Mac.
Use the Setup Assistant to select a Time Machine backup to restore from.


This method does not delete the existing user account, but simply creates a new administrator account alongside it and allows you to use the Setup Assistant to restore from the backup. 

You will need to delete the account the Apple Store folks set up for you if you don't want it anymore.

Good luck!