What have you tried and what’s not working?
Also just to clarify – do you mean to upload an image inside some page/post content?
If so you can use the Add Media button on the content editor screen to upload and add image files to your content. See https://codex.wordpress.org/Inserting_Images_into_Posts_and_Pages.
Or if you mean actually adding a logo for the site – If you’re running WordPress 4.5 and your theme supports the new site identity features, you can use the Appearance -> Customize to add a logo and a site icon for your site.
If you are not running 4.5 or if your theme doesn’t support this new feature, you may need to explore creating a child theme and adding a customized header template file and associated CSS. See https://codex.wordpress.org/Child_Themes for details on how to do this.
