The images in the carousel are located in Appearance>File Browser in the theme_assets folder.
They are labeled slide1.png, slide2.png and slide3.png
Upload new images to the theme_assets folder.
The images can be given any name but you may want to use a naming convention which contains a number (1, 2, 3, etc) at the end to make it easier to tell their order.
If you use the default file names (slide1.png, etc ), it is best to delete the sample files before uploading your new files.
Next go to Appearance>Template Editor and click on the Carousel snippet to open it in the Template Editor
Enter the name of the images you uploaded to replace the 'slide1.png', 'slide2.png' and 'slide3.png' names in the carousel code.
The carousel snippet includes a link to the Highwire support page (item 1). You can change it to the URL of your choice or remove it.
When finished, click Publish to update your store.