Simply start a new Career mode as a Manager, I went with all the default customisation options and chose an Authentic Club and just a random player (Zinchenko from Manchester City). Go to the Squad tab, then Squad Hub, choose your player, click Development Plan. Click RT to go to Position and simply chose a position (I chose CM). Now you must skip some time (took me 28 days), and in the Central tab, in the bottom-right hand corner you will see the Development box that shows your player's position change saying "Completed" with a green tick. Click this box, then click Y to confirm the position change.
I created this video to help:
Hope this helps.