I could use some advice on how to rotate a section of a UV map.

After using Poser for years, I finally decided to to have a crack at modelling. My first project is a coffin, and I'm happy with the resulting mesh, but I have a problem with the UVs.

The coffin has two groups "main" and "top" (lid), the two groups share the same material "main". The orientation of UVs on the top of the coffin run side-side, and those on the sides of the coffin run front-back.

I want to change the orientation of the UVs on the top only, so that they run front-back. How do I do this?

The tools I have are Poser PP2012, UV Mapper Classic, Roadkill 1.1, and Hexagon 2.5.1.79. I'm very new to the last two.