macOS

I got my brother's computer, and when i got it I switched to my apple id and icloud and I changed the computer password. I then wanted to change the user name to the computer and other changes using system preferences (such as the number linked to messages so i don't get all of his texts). I tried putting in the password and username to the computer into system preferences to change the computer username and it wont work. I changed the computer password, and the password that I have now works to get into the computer, but it doesn't work if I want to change anything in System Preferences, whats wrong?