The option for this is (from the menu bar:
From there, you can see your helper applications and, after selecting the one you want to change, the "change action" will be selectable and allow you to edit it.
If this is not available or doesn't work, then you have to get the very latest release. It's not available in version 0.6.1.
You need 0.6.1+ which you can get from here: