If you want to change a user's email, go to your profile settings. 

Then, go to the "email"-field, delete your current email and write in your new email. 

Once you have changed the email, you will see a warning, marked in red, that tells you that your email has been changed. Make sure that the email you have written is correct. 

Once you are done, click "Save". 

If you need to change your password, you can read more on that here.