Devotions & Respeccing Guide – Grim Dawn Basics Ep. 3
To completely reset all your devotion points at once, you need to find a Tonic of Clarity, which can drop from Nemesis bosses in Grim Dawn. Yes, you can respec (Note that this answer is based on to the current state of the Alpha (currently build 13 as I write this).
Grim Dawn RoT | How to change your 2nd Mastery with GD Defiler