You can cancel a production order in the Draft or Finalized stage by clicking on 'Cancel Production Order'. This means that you will not move forward with the production.
If you have already started production and wish to discontinue, you can click on 'Stop Production' (e. g., Broken equipment). This is as good as 'Completing' production in that you will be asked how many have been produced so far, and how many units of your components will be put back in inventory as available stock.