If you boot the Id into GUI mode (Option 1), the network manager is a bit easier to work with. This will require a mouse though. You can run option 13 from the main menu in GUI mode. Find the network connection you're trying to configure and click Properties. You can change the key there and click OK. Wicd should only save one entry per network connection, so once you save the new configuration, it will overwrite the old one.
To answer your question, you just need to hit 's' to save. However, the key field can't be selected when you do this.