If your file associations to .jar file are correct you should be able to just open the client. Sometimes jar extension gets hijacked by other applications, winrar seems to be guilty of this often. You have two options you can either right click on the file -- open with -- java or change your file associations for jar files to open in java. There is a neat little program that we recommend called jarfix that automagically sorts it for you.
Feel free to message me in-game, on the forums, or on Discord with any questions or concerns you may have.