I suppose they were added from your Contacts app - those who also use Whatsapp.
In the app, in Favorites, there's "Edit" at the top left. When you tap it, you'll be able to remove the contacts you don't want to see there. What you have to do, is tap the red minus button to delete them from Favorites, then tap "Delete", which appears when you tap the minus button. When you've finished removing contacts, tap "Done" at the top left.
To add contacts, tap the + sign at the top right. This will open the Contacts app, where you can select the people that should appear as favorites. Tap one of those. The contact's information will be visible. Tap it, to add the person to your favorites.
Hope that helps.