You can set on your Samsung Galaxy S6 within the Phone app favorites. Favorites are contacts that you contact often. If you have created on your Samsung Galaxy S6 some favorites, it may now be that you want to reorder it.
If you do not know spontaneously how that works, then we want to briefly explain how to reorder the favorites in the Phone app.
Open on your Samsung Galaxy S6 the Phone app. Switch to the "Favorites" tab so you see all the favorites that you have added. To sort this new, tap at the top right on "More" and then "Reorder favorites".
You can now tap and hold individual contacts to drag it then to a new position. If you have your contacts rearranged, then tap at the top right on "Finish". Your favorites are now sorted on your Samsung Galaxy S6 in the desired order.