If you haven’t exposed any frames yet, simply change the camera to ISO 400 and shoot normally.

If you’ve already shot part of the roll at ISO 200, those frames were given about 1 stop too much exposure. With color negative film, that is usually not a big problem—negative film has enough exposure latitude that a 1-stop overexposure is often fine.

The simplest approach is to keep shooting and have the film developed normally. You usually do not need to tell the lab anything special.

If you want, some labs may be able to pull-process the film by 1 stop, but this is often unnecessary for just one stop of overexposure and may cost extra. Overexposed negatives may come out a bit denser, but often still print or scan well.

So in practice:

• no shots taken yet: set ISO 400 now
• shots already taken at ISO 200: don’t panic; finish the roll and process normally