you need a program called 7zip, once installed you need all the files that were splited from the original file, once you have all of them you need to make sure the files have the same name and the right end part (.001, .002, etc.) and you just right click the first one and use the option in 7zip section of the contextual menu to unzip the file (in case the files have different names or extensions or are in the wrong order, it will prompt an error). Be careful where you download the program from and which version you're downloading, 7zip is a program that have had several cases of malware injection in the past as it's not updated frequently.