Their work should double the energy of what's already the most powerful particle accelerator in the world. BBC News is the first to be allowed to see inside the LHC - on the French-Swiss border - to watch the work being carried out.
Scientists believe the upgrade will enable them to discover new particles which will lead to a more complete theory of how the Universe works.
A project leader with the LHC's Atlas experiment, Dr Pippa Wells, told BBC News that there was much more to come from the LHC.
"The past two years have been the most exciting in my time as a particle physicist. People are absolutely fired up. They've made one new discovery (the Higgs) and they want to make more discoveries with the new high energies that the upgrade will give us. We could find a new realm of particle physics."