There are a few ways to install glibc. You can install the libraries as a test, using the existing libraries as the default but letting you try the new libraries by using different options when compiling your program. Installing in this way also makes it easy to remove glibc in the future (though any program linked with glibc will no longer work after the libraries are removed). Using glibc as a test library requires you to compile the libraries from source. There is no binary distribution for installing libraries this way. This installation is described in Installing as a test library.
The other way described in this document to install is using glibc as your primary library. All new programs that you compile on your system will use glibc, though you can link programs with your old libraries using different options while compiling. You can either install the libraries from binaries, or compile the library yourself. If you want to change optimization or configuration options, or use an add-on which is not distributed as a binary package, you must get the source distribution and compile. This installation procedure is described in Installing as the primary C library.
Frodo Looijaard describes yet another way of installing glibc. His method involves installing glibc as a secondary library and setting up a cross compiler to compile using glibc. The installation procedure for this method is more complicated then the test library install described in this document, but allows for easier compiling when linking to glibc. This method is described in his Installing glibc-2 on Linux document.
If you are currently running Debian 1.3 but do not want to upgrade to the unstable version of Debian to use glibc, the Debian libc5 to libc6 Mini-HOWTO describes how to use Debian packages to upgrade your system.
If you are installing glibc 2 on an important system, you might want to use the test install. Even if there are no bugs, some programs will need to be modified before they will compile due to changes in function prototypes and types.