Go to the Wifi settings on your smartphone
Tap and Hold on the wifi network that you want to connect and after that you will see forget the network option
click on the forget option
and then try to connect to the wifi network
(make sure that you are entering the correct wifi password)
Authentication error comes when you try to connect with any network with wrong password, in your case may be old password is saved. You can forget that network and connect back again by using the new correct password.