switch your local environment to staging (your changes will not be lost) -  git checkout staging

ad 1. git pull --rebase
ad 2. if you did the switch, you are already switched to staging  - git checkout staging
ad 3. Commit your work and push to staging -
 git commint -m "Nice commit description"
 git push - this will deploy it to staging website

than test on staging. To deploy to live:
git checkout master
git pull --rebase
git merge staging
git push