You can edit the .dae file with a normal text editor as well, if you know what you're looking for. I recommend not using Notepad though since the line breaks will likely be wrong. Try Notepad++.
Then you can change the file name for which texture it will reference. Scroll down the file until you find "library_images" and within it you should see a few texture references. Edit those and save the file.
PS: You only need to edit the last part of the path. You don't even need to edit the extension, since the final extension will always be .mat. In your case, you only need to edit the "servant_grunt" part near the end to match your new texture file.
Then open the model in the ModelViewer to generate a mat file for your new material.