Once you login to the website, go to Smart Slider>dashboard on the left navigation bar. 

Hover over the Homepage Slider. Select Edit.

Duplicate one of the slides by clicking the three dots.

Open the new duplicated slide. It should look like this:

Click the + in Thumbnail in Content and upload a new image or select from the Media Library. Also, if desired, change the link to the website page, the slide title and description.

Also, click Style and click the + to upload the new image. 

Click Save. You can unpublish the old slider by clicking the three dots. Select Unpublish.