Updating the gem itself is easy:

sudo gem install haml --no-ri

This is how I updated an existing Rails application which had an existing copy of an older version of Haml "frozen" into the vendor/plugins directory:

# blow away old gem
git rm -r vendor/plugins/haml

# clone the official repo (seeing as we can't just "svn export" anymore)
git clone git://github.com/nex3/haml.git vendor/plugins/haml

# make sure we have the right commit checked out
(cd vendor/plugins/haml && git co 2.0.0)

# blow away unwanted metadata
rm -rf vendor/plugins/haml/.git

# take a snapshot
git add vendor/plugins/haml

For info on setting up aliases like git co see "Git quickstart".