You can't edit an activated App without de-activating it, which can present a problem if your App is being used regularly (you don't want your App to be unavailable while you edit it). Even if you don't mind de-activating your App temporarily, it's best practice to never edit the current, activated version of your App directly (you might make a mistake and then spend frantic hours trying to revert your App back to its original condition).
Here is what you should do:
This allows you to keep your original App "clean", free from edits that may break it. Only when you are satisfied with your edits should these changes be merged with your activated App pursuant to step 3...
In the App Builder for your activated App, go to Settings > Code and look under 'Replace by copying'; find the App you recently edited:
