We do not recommend changing your pregnancy onboarding information since it involves removing your pregnancy in the app entirely.
However, you may do so, just be aware that it involves quite a lot of steps. Here's how it works: go to your profile and change to trying to conceive. Enter that you gave birth or experienced a loss. Once you're back in trying to conceive mode, go to your profile and remove the previous pregnancy. You can now switch to pregnancy mode once more, and enter the requested onboarding information.