There are many different problems you may be facing that don't allow you to change your password. Please, check the links below to see if you find the exact issue you are experiencing:
If you want to change your email address, and you can log into your account: How do I change my email address?
If you forgot your password: I forgot my password. What do I do?
If you can't log in to your account in order to change your password: Why can't I login or sign up to MyHeritage?
If you changed your phone number, and aren't able to receive the verification code for the Two-Factor Authorization: I changed my phone number, and now I don’t get the authentication code. What can I do?