Forcing the wifi password entry dialogue to display
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).