Its very simple just follow below link...
Please don't forget to refresh cache.
To remove the "Luma Logo", follow these steps from the DevDocs:
- On the Admin sidebar, go to Content > Design > Configuration.
- Find the store view that you want to configure and click Edit in the Action column.
- Under Other Settings, expand Expansion selector the Transactional Emails section.
- To upload your preferred Logo Image, click Upload and select the file from your system.
- In the Logo Image Alt field, enter alternate text to identify the image.
- Enter the Logo Width and Logo Height in pixels.
- If you simply want to remove the logo, then enter "0" for the width and height. Click "Save Configuration"