I am an engineering student who spends most of his spare time watching TV rather than coding. So basically I have zero experience with any kind of version control system. My
git clone your-url local-dir
to checkout source code;
git pull
to update source code in local-dir;
If you wish to update several git repositories in one command - i suggest that you read a little bit on repo.
About updating the repository, you can do it by:
git fetch
git rebase origin/master
OR
git pull --rebase
For more information about using GIT you can take a look on my GIT beginners guide
You might want to start with an introduction to version control. This guide is specific to subversion, but the core concepts can be applied to most version control systems. After you have the basics, you can delve into the git guide.
To answer your questions directly rather than pointing you at documentation:
1) In order to keep it up to date, do a git pull and that will pull down the latest changes in the repository, on the branch that you're currently using (which is generally master)
2) I don't think there's something (widely available) that'll do this for you. To update them follow 1) for all projects.