Randy, there is nothing second-rate about the Xenar lens. It is a Tessar-design, and every bit as sharp and contrasty (if not more so) as the Zeiss Tessar that F+H used interchangeably in the Rolleiflex. They aren't quite as sharp in the corners as the later Planar and Xenotar lenses, but they are amazing optics in their own right. Do not knock that Xenar.
The lens can be removed by unscrewing the bezel in front of the lens. My understanding is that the viewing and taking lenses were paired at the factory, and so it probably is not advisable to replace it if you can avoid doing so.
Peter is correct, that you would install a fresnel (Rollei's version, designed for the camera, is called a Rolleigrid and is available in the used market) on top of the ground glass. Or you can get a replacement screen from Bill Maxwell, that will be brighter still.
If the camera is worth the effort, you might consider sending it out for a proper CLA. There are a number of threads here about reliable people who service Rolleiflexes. Replacing the screen on an older Rolleiflex, or removing and replacing lenses on the camera, risks throwing focus off, but can be quickly and economically done properly by someone who knows what they are doing.
Sanders McNew