what do you mean? you have a bunch of parts and you want to make it one file? cause if so, just extract part 1 (or sometimes part 0) and itll put itself all together for you.
when you select the folder and go to "add to archive..." after right clicking then you can select the compression capability (like from worst to best compression) and then under that you can say how big you want each part to be and then it will stop them at that point and split it...
