If at any point you need to add or remove hashtags from an existing campaign, head over to the campaigns section, click on the campaign you wish to edit, then finally click on 'Settings'.
This will bring you to the Campaigns Configuration page. In the 'Media collection' section, scroll down to 'Collect media with hashtags' and append any new hashtags to collect from.
Note: The hashtags need to be separated by commas and you should NOT add the Hash symbol ('#') to each word.