You can right click a terrain in the viewport, and under ‘transform’ you can set lock/unlock so you can move the terrains again, but this is not the correct method because it very difficult to line up the terrains.
You are supposed to be able to move the tiles in the ‘world composition’ window, where all the tiles should line up, and snap to each others edges. However, my tiles are all stuck together in one place, and move back to the center every time I move them, so I’m not sure how world composition works exactly.
Were you able to import the tiles correctly so that they are automatically arranged to create your landscape?