If you want to change the image in Section 2 (Welcome to our Website),
First add new image in Media Library
Go to Media -> Add new and then copy the File URL of that image
Then in section 2 change the image source with new uploaded image path.
To add image in WordPress page, just edit that page and click on Add Media Button to insert images in pages.
Please see the documentation, it will surely help you to customize the theme.