It's very simple to change the first name, last name, birthdate, or gender for yourself or a profile you've created for someone else. Follow these instructions:
From the Home page, tap on the avatar (circle with initials in the top right.)
Tap "Manage" next to the profile you want to change (Tip: If you don't see the "Manage" button, try going to that profile and then going back to this screen again.)
Change the information you need to, and then tap "Save" below the Gender field.
That's it!