I have version 6.0-4ubuntu1 installed, but here ftp://ftp.info-zip.org/pub/infozip/src/ i see only version 6.0.
If you want the current version (that you have installed) first check what package we're really talking about:
$ dpkg -S `which unzip` unzip: /usr/bin/unzip
Not all binaries belong to a package of the same name. This one does.
When you know the package, you can download the source that went into it, like so:
apt-get source unzip
Of course that requires you to have the source repositories enabled but that's simple enough to fix if you don't.
To compile it you'll need
unzip's dependencies. Again,
apt-get can help:
sudo apt-get build-dep unzip
And then you can follow the standard instructions for the application you're compiling (usually some form of
make) but you may wish to package it before installing.
checkinstall comes in pretty handy here as a quick package builder. See this for more: