In the Search bar, search for "System Preferences" and click the result

In the Preferences search box, search for "Portal Logo".

Searching should highlight the location of the relevant preference. Expand the results to find what you are looking for.

Click the Edit button.

The Logo editor will now appear. Click the Select button to add a new Logo.

You will need to navigate to the location of the image on your computer that you want to use and then select and open the image.

The selected image should now appear in the Portal Image window.

If you want to edit the Logo, click More > Edit

The Image Editor will now appear where you can use the provided tools to make changes. When finished, click Save.

When finished, click Save & Close

Then click Save & Close in the Preferences window to complete all of the changes.