How can I add new columns to a model after I creating it?

I might just be confused, but it looks like in Metabase, there is no way to add new columns to a model after it has been created.

Specifically I go to: Collections -> "Our Analytics" -> (click on model name) -> (click on edit query definition), and it looks like I can add new filters, but I can't add new columns.

In the Model query editor, you should be able to click on the drop down next to the model name and select the column you are interested in using the checkbox.