1 Create a new folder, eg: C:\GitHub\mynewbranch
2 Drag it into GitHub, fill the description fields, uncheck "Push to GitHub" and continue.
3 Open your "Repository settings":
And fill the "Primary remote (origin)" field with your repository url:
4 Click "Open in Git Shell" (below "Repository settings") and type this in the command line:
git remote set-url origin git@github.com:<username>/<repository name>.git
5 Then this:
git pull origin <branch name>
6 Switch to the desired branch in GitHub, enjoy :-)
P.S. to update future changes from the master:
git pull origin master