You've probably installed the campaign into your 'documents' folder... I did.
.
Once you've found the 'campaign_gb' folder, copy and paste the contents into C:\Program Files (x86)\Steam\steamapps\common\IL-2 Sturmovik Cliffs of Dover\parts\bob\mission\campaign\campaign_gb\
...or run the installer again stipulating the above path.
Up to you if you want to back up the original contents first. I think the majority view is that losing it would be no great loss.
