I have the same question, but it was not answered. One person referred to the Edit menu but I don't have that in Viewer 2.
Another said to hit Save at the bottom right of the Groups sidebar.
But when I change the title of Owner to something else, the Save button is not visible at the bottom. It's a plain black bar.
When I click in a checkbox to give myself a new role, I can't click anywhere else, there is no Save, and all of the Titles disappear from the main Second Life menu bar! It goes black. No Me, World, Build -- plain black.
Is this the same question or problem, or a different one?
I am on a Mac and this is the first group I have managed in Viewer 2. Grrrr...
Update: I went into SL with Imprudence and was able to make the changes with no trouble. I have no idea why Viewer 2 refused to let me change role titles. Or rather, I have no idea why the "Save" button disappears and you are stuck in editing mode till you force quit.