How to change images without removing and re-uploading via GUI back end?
Derivative images (the files that result from using Image Styles) are auto-generated; you need to replace the original image file, then flush the images already created for the style(s) you're interested in, so the derivatives be re-generated next time they're requested.

For the example path you've given, the originals will be in the root of the public file system (/sites/default/files), so just replace those with the new images, and flush the styles that use them.

Probably the easiest way to do the last part is to install the Image style flush module.