You can't specifically uninstall Cortana, but you can turn it off. Launch Cortana and edit the settings (in the Notebook tab on the right). There should be an option to turn it off. Once you do that you will still have a search box, but it's just a regular search box, not linked with the features of Cortana.
You can remove the search box by right clicking on the task bar and changing the options for the display of the search box or Cortana.