You can reset your password by clicking on "Forgot your password?" on the login page. You will need to provide the email address that is linked to your Strava athlete account on the following screen.
After submitting your email address, you will receive an email with instructions and a link to a page where you will be asked to enter a new password.
(If you signed up via Facebook, you will need to enter the email address that is listed as the primary on your Facebook account.)
Need to change your password?
After logging in, go to your Settings (the drop down menu under your name at the top of the page) and on the right click "Change Email & Password." After you enter your old password, you can then choose a new password.