How do I get more harder Arm Muscles and how can I have fast agility?

Well look you should do some real exercises 6 kg is nothing, i suggest you go to the gym and you cant just do any exercise asc workers there to write you a program, IMPORTANT you must not work the same muscle groups every day so if one day you are really working your arms you must not do the same thing tomorrow or you can f*** up everything, i suggest you go to the gym every second day that's the way i do it. And important DIET this does not mean eat less just change your meals the best thing is to go to a nutritionist, so if you hold to your nutrition and workout program you should see fast amazing progress. Good luck!