All edits made to your blog will be made inside your blog dashboard. Once there, you can change your blog’s theme by selecting “Themes” from the menu.
To edit the settings of the them you have chosen, you will want to select “Theme Settings.”
After you have selected “Theme Settings”, you can adjust various aspects of that theme in the menu on the left side of the page. These aspects will vary slightly from theme to theme, however these will typically include edits to the site logo, tagline, colors, typography, etc.
Inside the Theme Settings, you can see a preview of your blog's appearance before you publish it to the public. This will allow you to see the changes you make right as you are making them.
Always be sure to click the blue "Save & Publish" button at the top of the Theme Settings menu.