Changing or adding genres is not possible once your campaign has launched. They can't be changed because your song is sent out to all of the curators on day 1 of your campaign and there is no way to reverse this process.
Targeting different genres requires running another campaign targeting and sending the song out again.
How to target additional genres:
If you want to target additional genres, you can do this after your campaign ends by setting up a campaign rerun. This is a great option for expanding your reach and building on the momentum of your first campaign.