1) Go to SP Page Builder -> Choose "Pages"

2) Next, choose the page you would like to edit. For example, About Us page. Then click on the title to edit

3) At the page, go to Row Options by click the settings icon

4) Next is go to Style tab on the Row Options

5) Scroll down the style, you will see the the background image. Choose the image button to upload the new one

6) Upload the image file from your computer, then click insert 

7) After upload the image, click the button 'Apply' on the bottom

8) Last, save the page by click the 'Save' button on the right top