In order to keep both versions, I just changed the value of PATH environment variable by putting the new version's git path "/usr/local/git/bin/" at the beginning, it forces to use the newest version:
$ echo $PATH
/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin:/usr/local/git/bin/
$ git --version
git version 2.4.9 (Apple Git-60)
original value: /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin:/usr/local/git/bin/
new value: /usr/local/git/bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin
$ export PATH=/usr/local/git/bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin
$ git --version
git version 2.13.0