How can I change my GSM antenna?

This concerns me because it may be the “No Service” issue. You listed it as the model A1660. This model has a known issue by Apple due to hardware issues with the phone. A chip on the board somewhat breaks off it’s solder points and causes a “No Service” message at the top left. Is that what it is saying? If so, it may be the No Service issue and I would contact Apple. If it says you have service but you aren’t able to make calls, that may be an issue with the SIM from your carrier or an antenna like you suggested. If you can provide us with more info we can try to help you figure out what your next step should be!