First find y'er terrain.cgf and select it, then right click the file. This tab should pop up, giving ye many options. Select the one that m'mouse is hovering over.
(http://i1255.photobucket.com/albums/hh623/RosieTheFox/PicC_zpsyuzm8fvb.png)
Now another page should pop up, giving ye four or so tabs-depending on the file, the number of tabs may vary. Click the Previous Versions tab.
(http://i1255.photobucket.com/albums/hh623/RosieTheFox/PicA_zpseq1hsp3t.png)
As ye can see, this is not a terrain.cgf that I be using f'er an example. The reason why is that no actual different versions are supposed to be found. But, if ye do 'ave a different version to select. Do click it and click Ok/Apply.
(http://i1255.photobucket.com/albums/hh623/RosieTheFox/PicB_zps4uk8txr0.png)