Login using your existing username and password.
Go to Change Your Password (This is found to the top right of the admin by hovering over your account name)
Enter your old and new passwords and click Change
Your admin password has now been changed.
If you have forgotten your password, click on Forgotten Your Password on the login screen to retrieve it via email.
For additional security:
You can also set up multi/two factor authentication via the Your 2FA Settings link under the Change Your Password link.