Problem with Blurb in Lightroom

Hi all, 

I am working with Lightroom Classic version 6 to create photo albums in Blurb. I finalized a big project and when I exported the PDF file from Lightroom to Blurb servor, I received a technical error. After some research, I think it is because I am using an old version a Lightroom CC, which does not encompass the newest updates of Blurb (paper sizes etc). Unfortunately, my Mac is too old to support the newest version of Lightroom CC !! So my question is : how can I retrieve and copy the saved book I made with Lightroom CC on my Mac to my PC, which supports the newest version of Lightroom and Blurb ?..... 

Many thanks for your answers, this will save weeks of work ! It is a present for my daughter's 20th birthday, and I feel desespate if I loose all this work.