When using a clip with a logo you are able to change what logo is being shown.
Drag your clip to the timeline and click it to go into the edit window.
Below the preview and mini-timeline you'll see a logo.
Below it you'll see a checkbox marked Use original. When clicked you'll see the original logo being use.
To use your own logo, click the icon named Replace Image and upload your own image. At the time of writing the dimensions for a logo is 700x400 pixels .png file with a transparent background.
This is to standardize the logo input files to make sure the image that you're uploading will look as good as possible!
Your custom logo is not a live preview update and will be added once the project is rendered.
Ie you'll not be able to see your own logo on the specific clip or when previewing the entire project. This change happens when the project goes through its final export process.