Your campaigns will be sent to all contacts by default. To send a campaign to a specific group of your contacts, you will first need to add those contacts to a group.
Need some more information on adding contacts to groups? Read up here!
Once your ideal campaign audience has been added to a group, navigate to Automated Marketing:
On the Automated Marketing page, find the campaign you need to edit and click "Configure".
On the Configure Settings modal, find the campaign audience dropdown next to "Sending To"
To change the audience, click the dropdown menu and select the ideal group. You can also choose to send your campaign to all contacts.
Click "Save" and voila, your campaign's audience has been updated!