If you have started a link from a Relation card (other than a previous/next marriage), when you get to a People card, the "Link Back" button will appear. This permits you to link this card back to the parents or children. You will be asked if you want to do this or to cancel. Cancelling allows you to find or create another card before linking back.
Clicking on the "To Parents" or "To Children" buttons, if this card is not already linked, allows you to start a new link to a relation. You are then asked if you want to create a new card, find an existing card, or cancel.
When you get to the relation you want, click in the Parent (if you originally clicked "To Children"), or the Child (if you clicked "To Parents") to finish the link.*