First, make sure your image is the same size (height and length) as the original image that you are replacing.
In your administration panel, go to Content > Content Pages and edit
any content page you like (it doesn't matter which page as we're simply
trying to open the image gallery, not edit the page contents).
When you reach the WYSIWYG editor, find the icon to upload images. It
will open a pop up window, now click the browse button. It will then
open the image gallery window.
Find all of the images related to the slider in one of folders on the
left. (they may be in a sub folder). Note down the file name of the
image you will be replacing.
Use the upload button above to replace the images with exactly the
same file-name/s (e.g. if the first image of the slider was called
"image1.jpg", it must be the name of the new image that you upload into
the same folder). Ok and save through until the end.
This should now be done for you. However you may need to clear your
cache or restart your browser in order to see the live replacement.
