Modify Submodule and Compile ArduPlane

I cloned ArduPlane 3.8.11 to local directory with this command;
git clone --single-branch --branch ArduPlane-3.9.11

I want to change the address where the mavlink library added as submodule is downloaded.
I modified url of the mavlink in .gitmodules and .git/config.Then I applied git submodule sync and git submodule update --init --recursive commands respectively,but it does not work.Can I change the repository where the submodules in the ardupilot are downloaded?If the answer is yes how ?

The reason I want this is that I am adding to new messages to the mavlink library and then compile the ardupilot code again for vehicle.After every change I make to mavlink, when I do git submodule update, I want the changes to come to the ardupilot folder.Than ı want to compile ardupilot code with updates.