Hello! To answer your question, you need to click with Right Mouse button default, and drag it to the province you want to attack/move your troops to, it will snap to that province, you release the button, you get an arrow from your province to the province you transport to, there is a circle, you can move that circle around on arrow to chnage percentage of attacking troops
I will add a move army button in province menu in the next update! Have fun!