Make sure the logo has a transparent background. This is possible in Photoshop. Save the logo as a .png image.
- Go to the theme store
- you see an overview of different themes
- click on edit on the theme that is activated
- go to "header"
- tick: "Show the logo in the website header"
- with logo you can upload an image, first select the file and click upload
- Click on save at the bottom op the page to save the changed theme