I was recharging the A/C in an 04 Ford Mustang. When I went to recharge, I noticed the valve was leaking. So I went to Autozone and they gave me a replacement kit. However, the tool in the kit would not turn the core valve to remove it. I went back and they said there is only one size valve for all models. My question is am I doing something wrong? What is the proper way to replace this? It seemed easy.

Always safely and properly evacuate your system, prior to working on it. Never add refrigerant to the high pressure side! This link may help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-9.html#post1864159

I don't know why the tool won't work. Try a pair of needle nose pliers.