How do I edit / add details to an imported terrain in unity ?

Hi ...

I`m very very new to unity , and are trying to create a locations based mobile game prototype , that is based on real terrain data. Its a type of a Mixed reality app, were the players will navigate in the real world, using the game. Obstacles, houses and the environment that is in the real world would match the one in the game. But with added difficulties and items that are only in the game.

I have created the terrain using Cheetah 3d and scaled it up, so its matches the actual place that the the the game will be played at, and added a first person controller.

I have added a mesh collider and a basic texture to the terrain, but I now need to make the terrain more realistic, and add details like trees, river, and a riverbed.
I have watch several tutorial online, and poked around in the unity forums and the answers, but i can't seem to find any thing that seems logical to me =/

I would really really appreciate the help !