You can change your username on your administration page at "My account". Confirm the changes by clicking "Save settings" at the very bottom of the page.
For security reasons, it's advisable to change your password every 6 months.

Please let us know if your email address changes. Select "My account -> Contact data" and insert your new email address in the relevant text field. Confirm the change by clicking "Save settings". You will then receive a confirmation email at your new address. If you did not succeed in changing your email address, you would also be notified.

Your password protects your privacy and ensures exclusive access to your weblog. It's not advisable to share your password with anyone else, because that gives the person power to edit your weblog and view any saved private entries. Your password should consist of at least 6 characters and can be a combination of letters, numbers and special characters.
It's best to change your password every six months for security reasons.