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 in the navigation pane on the left side of your dashboard.
On the Automated Marketing page, find the campaign you need to edit and click "Configure":
On the Configure Settings modal, you can choose who will receive this campaign. "Send to all contacts" is the default selection, but you can change this to send to a specific group by clicking "Send to groups" and selecting the group you would like the campaign sent to.
Click "Save" and voila, your campaign's audience has been updated!