Changing Passwords

How to change hostmaster@, postmaster@, admin@ and abuse@

Those should be automatically configured as aliases pointing to the account that is designated as the MiaB administrator.

I forwarded an account to one of such and need to access it. How do I change the password after the system has created them?

It is only an alias, so you cannot log in with it. Emails sent to the addresses will be forwarded to wherever the alias is pointing. If you want to send as one of the addresses, you can do so using a client where you change the sender address to the alias when sending mail using the account the alias is forwarded to.