Download and install the free 7-Zip software (or Keka if you are on a Mac).
All parts must have the exact same name before the numbered extension (e.g., Swift_Chubby_Engine.7z.001 and Swift_Chubby_Engine.7z.002 ).
Select 7-Zip from the menu and click Extract Here or Extract to "Swift_Chubby_Engine" .