Wireless Arduino (Atmega 328p) sketch uploading through Xbee S2

Hello my friends, I am working on a project where wireless sketch upload into remote Arduino (Atmega 328p) is needed. I have tried to use this tutorial: [login to view URL] But Arduino IDE throws "avrdude: ser_open(): can't open device "\\.\COM6": Access is denied." when I hit "Upload" in Arduino IDE. Maybe I have Xbees set up wrongly, I could not find arduino settings file in that tutorial attached (author said it is attached). But I don't know, problem might be somewhere else. Do you think you could help me to make this work? Thank you!