How to change banner/logo in servicenow

To update the banner image in the System Properties > System page, you must first upload the image.

Note Note: Setting a banner image in the company profile overrides the image selected in System Properties > System.

Go to System UI > Images.

Click New.
Enter a Name for your image. Valid names must end in .gif, .png, .jpg, or .bmp.
Click the Click to add link in the Image field.
Click Update.
Go to System Properties > System.
In the Banner image (must be uploaded first) field, enter the file name of the uploaded image.
Click Save at the top of the page.

Source: https://wiki.servicenow.com/