When you first make a SmartLoop, you'll be able to select a group of Default Boards that your Pins will publish to. Any Pins added to the loop will be set to publish to the default boards of the loop they're added to.
Updating the default boards will not change Pins already added to a loop, however.
There are two ways to update the boards for a Pin that is already in your loop.
The first way is after making changes to the default boards in your Loop, simply remove the Pin from your loop, and add it again. The new list of default boards will be applied to the newly added Pin.
The second way is to find the Pin in your loop and edit the boards in that Pin individually.
1. Select the loop that you'd like to edit.
2. Find the Pin that you'd like to change and click the area with the boards
3. Add or remove boards from the looped Pin, then click Confirm Boards