Your Facebook login is sensitive information--if another person gains access to your account he can leave comments or send emails in your name. If you think another person might have figured out your password, err on the side of caution and change it. The email address you use as a username during the login process can also be changed quite easily. When you change the email used for your login, Facebook will begin to send all communications to that address.
Change the password associated with your login information by clicking on the "Account" link in the top right corner of your Facebook screen and selecting "Account Settings" from the menu.
Click the small blue "Change" link in the "Email" section of the page. If you want to use one of the alternative email addresses previously saved to your profile, click inside the circle immediately to the left of it and then select "Change Contact Email." To use a new email address as your login, enter the new address in the "New Contact Email" field. Once the email appears in your list of addresses, click the circle next to it and then "Change Contact Email."
Click the "Change" link in the password section of the "Account Settings" menu. Enter your old password in the appropriate field. Select a new password with at least six characters in the "New Password" field. Confirm the new password in the last field and then click "Change Password."