The easiest way to reset Wi-Fi settings is to delete Wi-Fi related configuration files. Please note that by resetting network settings you will need to re-add and re-join any networks and enter the passwords again.
• Disable Wi-Fi by pulling down the Wi-Fi menu and choosing to toggle the Wi-Fi switch Off
• Go to the Finder, pull down the "Go" menu, choose "Go to Folder..." and enter the following path:
the click the "Go" button
• Select and delete the following files from this folder:
• Restart the Mac by going to the Apple () menu and choosing "Restart..."
• When the Mac boots back up, enable Wi-Fi again by going to the Wi-Fi menu and toggling it back On
• Select the network you wish to join
This approach will often resolve many wireless networking issues encountered on the Mac, but if you're still experiencing any Wi-Fi networking issues, you might want to reset the router and the modem itself, which typically involves unplugging those devices from power source for 10-15 seconds and then plugging them back in again, then waiting a few minutes for those to rejoin the appropriate networks.