Open Keychain Access (in your Utilities folder in your Application folder).
Use the filter in the type right to type in the name of the wifi you want to forget.
Right click on results with the network name and choose Delete "[network name]."
Use the wifi menu to connect to the network again and you will be prompted for the wifi password.
Alternatively, follow these steps. Beware: the network names aren't searchable and your network name may not appear in Network settings if you have not successfully completed connecting to it before.
Open System Preferences (in your Applications folder).
Select the Network pane, and in the left-hand list of sources, select Wi-Fi or AirPort (this differs in different versions of OS X).
In the lower-right hand corner, click Advanced…. A pane will pop out, and in the list of tabs at the top, either Wi-Fi or AirPort will be selected (depending on OS X). This is the leftmost tab; if it isn’t, then click on it.
You should see a list of Preferred Networks. This is a list of all the wireless networks that your Mac knows about, and thinks that it knows passwords for. Select the network which has a changed password in the list, and press the little - button which is to the lower-left hand corner of the list.
Then press OK in the lower-right hand corner of the pane, and then Apply in the lower-right hand corner of the System Preferences window.
This “forgets” the network. The next time you ask your Mac to connect to that network, it will see it as a new network and throw up a password prompt for you. Once you enter the new password, OS X will treat it as before (logging in and connecting automatically for you).