You will need to take the following steps to reset the Discovery Plus app:
Visit http://www.discoveryplus.com/gb and click on login on the top right corner of the page.
On the login page, click on "Reset password."
Enter the email address to which you have registered on discovery+ to receive a reset password email.
Open that email and click on the reset password link.
Enter a strong password (which contains at least 8 characters, of which at least one capital letter, one lowercase letter, and a number).
Click on Update.