@walshse, it depends on what kind of changes are desired. To hide something like Proudly powered by WordPress would be something relatively easy to do with just some CSS. However, if you want to actually make changes to the content, better to do so through a child theme. The “simplest” themes, without a lot of options, are usually the ones which cannot be customized so easily except through a child theme. Some themes have more options available for customization, others do not.
If you go to Settings → Discussion from the admin dashboard, you can uncheck the box labeled Allow people to post comments on new articles and that should get rid of the “Leave a reply” on any new posts. To remove them from existing posts & pages, go into Posts → All Posts, click the Quick Edit link for the individual post, uncheck the box labeled Allow comments and click the Update button. You can do the same for your pages by going to Pages → All Pages.