Yuki P. best practice is to duplicate the Relume figma kit and make that your source of truth and only design file - embrace the already built in design system, variables, components, etc and then add to that file however you see fit.
Be careful what you edit within the style guide page itself in terms of removing or moving things around as the Relume figma plugin expects certain elements to stay where they are.
But if you wanted to add new pages or create new components or new variables, text styles, etc - all those kinds of edits will not break anything. So yeah, just embrace as much of the already built kit and you'll be off to a good start!