Zwift takes all your physical measurements into account for features like leaderboards, speed, and how you're displayed in game, so it's always good to keep your profile updated for the most accurate experience.
Note: you will need to create your account through the game first before you can edit your profile details.
You can access your User Profile menu from the Menu Screen from within the game. It's located right underneath your name.
You can always change your displayed name, birthdate, country, gender, height, weight, and FTP from this screen as well as switch between Imperial and Metric units. It will also display your current FTP as set by Zwift (either automatically or via our Workout Mode FTP test).