Umm...I have never played B2 so I wouldn't know, but the general idea is to first find the save files for Steam B2. Generally, it is either in the program files where you installed the game or it is in the AppData folder which you have to get to by first going into folder options and click the option to show all hidden files, then go to Windows (C:), Users, Your PC Username, and AppData. Normally, the saves for many games are in the Roaming Folder after you open the AppData folder. It is also highly possible that the save files are elsewhere (like your Documents), but you will have to do some digging. If you can find the save files, you just need to copy and paste those save files to the place where B2 Epic Games Store version store their saves. Theoretically, this should allow you to move/copy savegames from Steam B2 to Epic B2.
