Left Click the red cross, this is the fonction for fixing parts.
Left Click on the part you want to remove. (the one you don't want to print again)
Left Click on remove selected triangles. The part will disappear.
Left Click on apply now, and in the open box, choose "YES"
Select the tab "Part" and export as STL
Left click OK. The modified part will be placed in the same folder than the original STL, with the name "Index3(repaired).STL"
That's it you are ready to GO!