如何修改以前的git分支

I have 3 git branches - Develop, QA and Master. Developers use the Develop branch for all development. Once a task is completed by a developer and the code is committed to the Develop branch, the commit is merged to the QA branch for testing. After testing is complete, the code is merged to the Master branch then deployed. We just found a bug in the QA branch that must be fixed before deploying. The fix has been made in the Develop branch but also includes changes that we do not want to deploy at this time. So, how do we fix the bug in QA? My first thought was to make changes directly to the QA branch just to fix the issue then merge those changes back to the Develop branch. But I wasn't sure what problems that may introduce to the Develop branch (ie. lost code etc...)I have 3 git branches - Develop, QA and Master.