The grid is documented here<http://twitter.github.com/bootstrap/scaffolding.html#gridSystem> .
You can override CSS in the normal way - just include your changes after bootstrap - but dealing with the CSS directly is very much the wrong way to work with bootstrap: you should work with the LESS files and compile them to your own CSS files. The LESS files have a grid calculation system built-in so changes to the grid are really straightforward (look at the bottom of variables.less for the grid settings) - doing the equivalent in the compiled CSS is *very* hard work. As far as dealing with updates, I handle that by creating my own version of bootstrap.less (the main less file that pulls others in) and my own variables file, then I merge any few goodies in from any updates to the main bootstrap.less (which is really just a list of includes) file, that way I only need to look at one file during an update.
