Connect to PC and then use Windows Explorer or other file manager program to delete what you don't want? If connected in MSC mode (standard default, if you don't change it to MTP in Settings) the internal and external storage of the J3 is just assigned one or two drive letters by Windows when you connect it to the PC via USB. So you can treat it like any other housekeeping function, such as deleting things you don't want.
Or, you can FORMAT the whole device (preferably using the SD Formatter utility
) and start over. The J3 will recreate all necessary system folders and files at the first re-boot after a FORMAT.
I'm sure much of what gets pre-loaded at the factory is MORE than what gets re-created after a FORMAT. I don't know about games since I don't use that functionality on the J3, but I FORMAT'ed mine to recover from some initial mistakes or mis-steps during my learning-about-it period. It's also an easy way to just "start over".