How do I choose an archetype

I've watched videos about archetypes but nowhere on the internet can I find how to choose one. Can anybody tell me how I choose one?

 

you can modify your ability tree by clicking the compass and then the blue icon on the left of the menu

if you were confused about the tree itself - you don't necessarily choose an archetype directly. the tree lets you unlock individual upgrades, and some of those are tied to one of the three archetypes; when people talk about using an archetype, they mean they've focused their tree on the upgrades corresponding to the archetype(s)