No idea what I’m doing – need to edit mobile version of site without changing desktop version

I'm sure this has been asked many times lol. But I do very basic web stuff and am editing a site for a family member's business.

I've tried WPTouch, doesn't work at all, I can't use Elementor because it seems to erase all my existing content. Is there an easier way for me to do this?? Explain it like I'm 5 please! Lol

Edit: grammar