Hello Karolina Koziejowska,
Thank you so much for using our theme MercantilePro.
1. Go to Appearance > Customize > Widget > AT Featured Page and check which page is selected for Featured page. After that find that pages on Pages > All Pages. After that you can edit the content on the pages. Add the content on the Excerpt field so that it will displayed on front page. Remove the Read More field empty to remove the Read More.
2. If you want to create the contact page like our demo, you can use add the HTML content on the page. You can use the dummy content so that you can make the theme as demo. Please find the content and instructions for theme here.
Hope, this will help you.
Regards