Forgetting a Wi-Fi network on macOS and OSX
Removing and re-adding a network can be a good fix for lots of connection problems. If you’re unsure of how to remove a network on a Mac, you’ve come to the right place.
Instructions
Click the Wi-Fi symbol at the top-right of the screen and open Network preferences. Select Advanced in the bottom right.
Select the network you want to forget and click the '-' sign.
Click OK and then click Apply. You have now forgotten the connection.