You can change your password at any time from your account settings.
When logged in and viewing your inbox, click on the user menu in the top-right, click on "My Settings". You will then be in the settings screen, find "Change Password" in the left menu and choose a new password.
Changing your password may log you out so you will need to log in again after a password change.
If you can't remember your password, it's easy to request a password reset and a link will be sent to your email address. From the login screen click on "Forgot Password" and enter your email address.
If your email address is not registered, you will not receive a password reset email.