Go to www.myheritage.com, click on 'Log In' in the upper right corner and then click on 'Forgot your password?'.
You will receive an email with instructions to create a new password.
If you need assistance in changing your password, please see this FAQ:
If you build your tree in Family Tree Builder, it will be necessary to change your password there as well.
In order to change your password in the Family tree builder please see this FAQ: