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

[Nicholas Allen]

> But it doesn't need that feature in order to be able to find the
> revision where a bug was introduced.  Revert will work just fine for that.
>
>   
I thought that revert with a -r argument will change the working tree so 
that committing would undo some changes. If you do this multiple times 
it doesn't make much sense does it? Maybe I'm missing something....

Nick