Tap on "Forgot Password" located under the blue "Login" tab:
(There are options to retrieve a forgotten password in on the "Sign Up" screen too.)
Once you enter your e-mail address and click "Forgot Password", you will see this screen:
You should receive an e-mail that looks like this in your inbox:
If you are not receiving the e-mail, check your spam inbox before contacting our customer support team (email@example.com).