In short, the hard way is to import it, then use PHPMyAdmin and edit the new style, copy the new stuff, then put it in the old style; otherwise you'd have to delete it and then import the new one and users would have to update to the new one.
Or, I guess you could import, then edit the old one and use the "use settings from another stylesheet" option...
Or, if its your defautl stylesheet, you could just install the new copy then set the new copy as your default...
