You don't. Bitrate is only an issue when you are rendering.
If you want to render to a different bit rate, as I mentioned in your other thread, you'll need to to to File/Render/MPEG2. From there, choose Custom, and set your bitrate.
If you aren't familiar with what you're doing, there is an easy opportunity to mess up. I'd recommend using the templates until you learn more about what is going on with the encoder.