Adobe Reader does not use user names and password. This if a functionality of the operating system. You need the name and password of an administrator. If you do not know it, then you need to ask the person that is the administrator of the computer. If you are the sole user of the computer, then you may need to reset the administrator name and password. How to do this varies by computer and operating system. Check the documentation that came with your computer.
For a update of Adobe Reader you don't need user name and password.