First, you'll need to sign up for the app. Then, you'll be able to see a Health Profile on your Me page (the person icon at the top middle on Android). Click that to go in and change your Birth Control method.
You may notice that your fertile window is no longer displayed after you do so. That's because Eve does not display fertile windows for users on certain birth control methods.
For example, if you're on the pill, the hormones you're ingesting are preventing you from ovulating, so you technically do not have a fertile window. This is why we remove the fertility information from the calendar view and do not send you notifications about it.