How can i use a starter theme and not having to worry about not updating?

I design my sites using the starter themes and the Upfront Builder, Usually I change all or several of the templates and layouts and change the theme name but every time i try to edit my site it ask me to update the theme and if i accidentally press in Update, all the changes i made to the templates are restored to the original starter theme and i lost all my changes....

I already tried to make a copy of the theme using the same Upfront Builder but it still happens anyway. Is there a correct way to do this? create a new theme using a starter theme without having to worry for the updates of the original theme?