Contacts that you publish under Publish > Contacts will be shown on the Contacts tab in your public newsroom. When creating or editing a contact, ticking the checkbox that says Show as press contact in your newsroom, will define this contact as a press contact. You can also use the three dots menue under Publish > Contacts to set or remove the press contact status.
An active press contact will appear on the front page of your newsroom. Moreover, stories that you create will by default automatically include your press contacts as related contacts.
Whenever you change to set different contacts as press contacts, these will update accordingly both on the newsroom front page, and as related contacts in your stories. This is also the case for stories that have already been published (unless the related contacts were manually changed when editing a material – in that case the individually set related contacts will keep showing, regardless of who is currently being defined as a press contact).