I do not remember any of my administrator authentication codes

I have just began re-using an older computer. I need to make administrative changes to all 3 administrative accounts but when I try, it tells me that "Authentication is required to change user data". I don't remember the passwords for any of these accounts. I know one of them does not even have a password to enter the account but the authentication is asking for one anyway. I don't know what to do to authenticate. Please help me.