when you want to create a new branch for your product, you need to develop on a new branch, with which you can develop very new feature for it.

first you need a base, check out to the branch and create a new branch

git checkout master
git checkout -b feature

add submodule or switch the branch or the submodule

git submodule add -b feature https://.....
" or switch the submodule branch
cd submodule-A && git checkout [-b] branch-name

commit your change:

git add submodule-A
git commit -am 'you commit message'
git push

reference

• stackoverflow
• komodoIDE