How do I update a working tree to an old revision?

[Nicholas Allen]

> It will.  You weren't proposing to commit, you were trying to find a
> bug.  For the branch, that's a read-only operation.
>
> update -r would make it impossible to commit anyhow, since the tree
> would be out of date.
>
> I don't understand how commit is relevant.
>   
I thought that revert was making a change to the working tree that you 
could then commit to revert to a certain revision. I guess it's just a 
misunderstanding of what revert -r does on my behalf. It sounds like 
what I would expect update -r to do. I guess I would use merge command 
to make changes to working tree that revert to an old revision then?

Nick