Adding content to HTML after implementing files into theme

Hello all,
Someone worked on a website and implemented the static HTML files into WordPress. There are things i need to add/edit such as divs and text. Is there a way i can go back into the HTML files, edit what i need to edit and then re-upload or update the HTML files in wordpress via php? I see the header.php, index.php etc… however, I know absolutely nothing about php beside the fact that it’s calling the HTML. I also tried to edit what i need in the customize portion of wordpress. Is it wrong to write HTML in the edit page panel? Please help. Thank you!