1. Under Apple icon open System Preferences/Network
2. Click "Turn Wi-Fi Off"
3. Highlight Wi-Fi entry on left
4. Click Advanced button on right
5. In the new window that opens, highlight the problematic network 
6. Click the "minus" button ("-") under that network
7. Click OK - that window now closes
8. In original window, click Apply
9. Click "Turn Wi-Fi On"
10. Now choose that same network from the dropdown menu and reconnect