Look into this tutorial by Roblox themselves
or in simple terms:
- Put this in the command bar, then press Enter
workspace.Terrain:CopyRegion(workspace.Terrain.MaxExtents).Parent = workspace.Terrain
- Save as a
.rbxm file the generated terrain under the workspace.Terrain
- Go to a different place and paste this in the command bar, then press Enter again
workspace.Terrain:PasteRegion(workspace.Terrain.TerrainRegion, Vector3int16.new(-32000, -32000, -32000), false)
then you should have it 
3 Likes