The current STABLE release of the game is available to be downloaded/updated in the launcher. When a new update comes, it will let you know you can update it there. HOWEVER, you can not download the dev/prebuilds from the launcher, you must do that manually. Currently, some people play the prebuilds and some do not. The new player model is in the prebuild, and the next update. The next update will come when the game is stable enough and free of most bugs.
So you will not gain anything extra RIGHT NOW for paying that $6. You can play the game, and prebuilds for free currently, and you should never have to play for a prebuild/unstable game here. But that $6 you paid is preordering the game. Why preorder the game? You get it at a lower price, and the price will rise over time.