In the app, at the bottom of the page where you sign in, tap on the link that says "Forgot your password?" Enter the email address that you used when you signed up for PlantEaters, and then click on the link in that email once it arrives. You'll be able to enter a new password. Note that the link in the email may only be used once, and expires after 24 hours.
You can also have the email sent to you by visiting http://getplanteaters.com/resetpassword.