You cannot update glibc on Centos 6 safely, but, you can install 2.14 alongside 2.12 easily, then use it to compile projects etc.
- mkdir ~/glibc_install; cd ~/glibc_install
- wget http://ftp.gnu.org/gnu/glibc/glibc-2.14.tar.gz
- tar zxvf glibc-2.14.tar.gz
- cd glibc-2.14
- mkdir build
- cd build
- ../configure –prefix=/opt/glibc-2.14
- make -j4
- sudo make install
- export LD_LIBRARY_PATH=/opt/glibc-2.14/lib