To change the logo in the header of the website
Classic Website Editor:
Go to the Website tab and click Manage
Click the Settings button in the upper right
Go to the Colors & Images section, find Logo and press 'Select Image'
Scroll to the bottom of the page and press Save
Website Builder:
Go to the Website tab and click Builder
Click on the Logo in the upper left or center of your banner
Depending on your template the logo will be in a different location
It will be automatically applied when you upload a new image and press Save, you won't need to press Save Draft or Publish
NOTE: It is recommended to use an image with a transparent background