In order to delete an app, you must first ensure it is not open.
To quit the app, bring up the app window and then press the CMD⌘ + Q keys together. If you are running a menubar app, right click on the menubar icon and select 'Quit'.
After that, open Finder and go to your Applications folder. Find the app you want to delete and drag and drop it into the Trash Can and then empty the Trash Can.