You can edit an HTML file with any text editor. Updating the sh3d file displayed by the viewer is explained in points 3 and 4 of the README.TXT file.

You can copy the viewer files with FTP but also with the tools proposed by your host provider. In that case take care of keeping the same organization in the lib directory or it won't work.

How do I go about changing my forum user name/ID to Mark?

Sorry you can't.