Exposure is reciprocal, that means if you add a stop of exposure somewhere, you have to take it from somewhere else. And vice versa. However film speed (ISO, ASA, DIN, whatever) is not absolute, it's a recommendation. For instance you could hugely under-expose your film (shoot everything at 1/1000 at f16) and if you equally hugely over develop your film, you'll probably get an image. Not optimal, but useable. The ability of individual films to offer this flexibility is known as their latitude. Generally speaking fast films (400 ISO and above) have good exposure latitude, slower films less so, although it isn't the same for every film.
Depending how automatic (or not) your camera is, compensating for the effect you want is easy, or difficult. The hardest would be a fixed aperture, fixed shutter speed point and shoot camera. Experience is the only guide to how much or how little development your film would need, relative to film speed and lighting conditions. Cameras with adjustments are easier, and vary depending whether they have match needle metering, LED indicators, or LCD readouts with + or - exposure settings. If it's an auto model, the simplest thing is to call your 400 ISO film 200 (pulling one stop) or 800 (pushing one stop). Either way you or the lab may have to compensate. The more you push or pull film, the more you have to compensate in development. YouTube has lots of videos on camera exposure.