Unfortunately it is currently not possible to edit a Pin which has already been submitted to a Community. You'll need to delete your submission from the Communities it was submitted to, then resubmit it with the correct description if you want to make a change. Follow the steps below:
Go to the Communities page (located in the pop-out menu in the left hand navigation).
2. Find the Pin you added to be deleted under the "All" or "Yours" tabs.
3. Hover over that Pin and a trash can button that says "Remove Content from Community" will appear.
4. Click the trash can icon and then click the red "Confirm" button to delete and remove the Pin entirely from the Community. Alternatively, click the grey "Cancel" button if you don't want to actually want to remove the Pin.
5. Submit the Pin to your Community again. Make sure to change the description to be exactly how you want before you submit to your Community again!