hm .. might be depending on your client (and so this post would belong to the according client-specific board).

however, i suppose with "library" you mean your "downloaded files" folder in the opposite to your "uncompleted downloads" folder?!? if so, the usual procedure would be to move it back to the "uncompleted downloads" folder and start a download with an identical filename that is larger than the existing file. most clients in this case start to "resume" that file then ...

hope this helps. if not, try to ask more detailed in a client-specific forum ...