Actually, there is a simple way to swap your save slots.
Disclaimer! After I tested it myself I have had no problems whatsoever. But as you are fiddling around with steam cloud files, you might end up with inconsistencies. Therefore, you should always backup these files before you rename anything.
With this in mind, I would recommend to disable Steam Cloud. You can do that by right-clicking your game in the steam library, go to "properties", to "updates", and remove the check mark in the "Enable Steam Cloud ..."-checkbox.
Now you have to find your save files. They are located in your steam installation folder (most likely C:\Program Files (x86)\Steam on Windows) in the subfolder "userdata[some id]\583470\remote".
To change the slots simply rename file_x.sav and file_x.sav.old to the one you want. After testing it myself don't be to confused about the fact that the middle slot is file_1.sav, left is file_2.sav and right is file_3.sav. Moreover, if you rename it to a file that doesn't exist yet, you can end up with both showing up ingame. Just remove the one you don't need.
You can write to me if anything is still unclear. And be sure to check out the reddit post about save files: https://www.reddit.com/r/TheEndIsNigh/comments/6pj1mo/where_is_the_save_file_located/
I hope I could help.
