When you create a search column in TweetDeck you provide the search terms and TweetDeck keeps the column updated with all the tweets that match those terms.

You can edit the terms for a particular column by clicking the Edit button that appears when you hover over the column header:

This will reveal a text field where you can edit the current search terms.

Click the Save button to save your changes and your column will refresh using the new search terms.

NB: If the search column you edited was based on a Twitter Saved Search, TweetDeck will also update that Saved Search on the Twitter.com website