Unable to change my WiFi password

I changed my wireless password in my router settings. Now I want to change the password on my imac desktop wifi but I can’t find where to update the password. I went to wifi settings but when I click on “advance” i don’t see an opotion to see all the previously connected networks to change the password. I also went to Airport Utility but the screen says “internet disconnected” and don’t allow me to click anywhere. Please help me how I can change my wifi password.