How do I customize SharePoint online header and add my company logo and change the look for my external site?
Click on Properties and Make sure you have header is selected
In page settings, include header
Now click on header to customize your header and select logo. Click on upload your picture and select your new logo and upload to SharePoint online site.
Once your image is uploaded, Click OK
Do you want to remove your header image and add something new? Click on Theme and select no Image
Do you want to add new style to SharePoint header? Click on Style and select the style you want
Customized header for SharePoint Online external site
Aryan Nava 