Amater needs some help

Hello I have just started with UE4 and i am doing on project just for fun. I want to change texture(grass to mud) of terrain at my ,river, but i dont know how… Can you help me with this ? And then i need some texture of roof ( rusty ) for free i dont know where i can find it. Thanks anyone for trying to help me.
Screenshots : http://prntscr.com/f4rkm3 , http://prntscr.com/f4rktc .

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Changing the texture:
Created a new material using the texture you want. Select the terrain element which you would like to alter. This will display the material currently applied to it on the panel on the right hand side. You can then switch it to the material you just created. OR you could edit the existing material and apply a new texture to it.

There are some very good vieo tutorials on Youtube (https://www.youtube.com/watch?v=sTAVY8qtGbg)

Free assets:
Unreal Engine comes with a lot of free assets. You can find and download more (free/paid) from the Marketplace (available from the launcher)

Ok i have done it step by step and the material is black … Do you know why ? Its completle same like on this video .

Post your material.

Make sure your material looks ok in the material editor before you apply it to terrain.

Also try rebuilding the lighting of your map.