Distinguish content from structure
Page content belongs to an individual page. A template controls the structure used to display a kind of content. Before making a change, check whether you intend it to affect one page or every page using that template.
Use the editor your theme supports
WordPress's Site Editor works with block themes and can edit shared layouts and styles. A classic theme may use other settings or a page builder. Start from the documentation for the theme that is actually active on your site.
Preview a shared change broadly
A revised header, footer or template can affect many pages. Review several representative pages after changing it, including one with long copy and one with very little content. Keep navigation and page identity clear in each case.