Go to MyHeritage.com. Then click on “Forgot your Password” in the Log In section.
You will receive an email with instructions to create a new password.
After logging in, please go directly to your profile to change your password to something you will remember.
If you build your tree in Family Tree Builder, it will be necessary to change your password there as well. Please refer to the following post how to change your email address and/or password in Family Tree Builder: How do I change my login in Family Tree Builder?