If your search is still running you need to hit 'Stop search' first and then you can change the search.

You can stop your search by heading to one of the tabs (keywords, location or platforms) and then clicking on 'Stop search'.

See below:

When you changed the search, for example removed a keyword, you can hit 'Update' to get the new search results.

If the search is not running anymore you can just change the search and hit 'Update' right away.