In the example below, we will be replacing the default "Add To Cart" button.

The first thing you will want to do is right click on the image you want to swap out.

Next, you will click on Save Image As and save the image with the EXACT file name it has on the website. Note, this is case sensitive. If you are trying to replace the image "AddToCart.png", and you upload "addtocart.png", it WILL NOT work.

Make the changes you want to the image and save it. Note, be sure that it is saved with the exact file name.

Next you will navigate to your admin and go to Content and Files > File Manager

In the Images folder, select the buttons folder within "theme"

Click on the Upload option to upload your new icon.

Choose select from the pop window and locate your saved image. Then click upload.

The image below shows the changed button.