To start with, add these rules to where your theme accepts custom CSS. This is sometimes in the customizer. There are also plugins that permit adding custom CSS.

.widgets_on_page h2.widgettitle {
  margin: 0;
}
.entry-content img.aligncenter {
  margin: 0 auto;
}

This may be good enough. If not, use the inspector to try out additional selectors. Adding .page-id-6 in front can be productive since it only occurs on your home page. When the added selectors affect the target area but not other areas, permanently add the selectors to the custom CSS. It can be useful to add a temporary rule that changes something more obvious, such as background color, so you are sure of what areas are affected. Remember that when you change or reload pages, you need to reapply any inspector changes unless they were added to the custom CSS.

Don’t forget to check the layout in all possible screen sizes, you may need to add rules within media queries to have the ideal layout in all cases.