The optimization of the headings of the theme

Hello everyone ! I encountered some SEO problems and was advised to optimize the headings of my theme. I love the theme catch box, I'm satisfied and I don't want to change it. How can I replace the h3 and h2 headings without changing the appearance of my site (my css): https://maviedecoeliaque.fr .... <h3> Main Menu .... <h2> Browse the articles .... <h2> 2 Replies to "name of the article" .... <h2> Leave a Reply Cancel reply .... <h2> Main widget area for sidebar .... <h2> widget title Sorry for my English, I'm French. I am a beginner in coding. Thanks a lot for your help ! Best regards ! Mathilde