If you created a poll in a group, you can edit it before people start voting on it. Click on the three dots next to the poll, then select "Edit." From the same menu, you can "Delete" the poll entirely. If you've created a poll using Stories, you won't be able to edit it. You'll have to completely delete the Story, then start over again. This also applies if you want to delete the poll from your Story – you'll need to delete the entire Story. If you used a third-party app to create your poll, then you can go back at any time and tweak the questions. Your changes will be updated in the cloud.