Work has already started and will end on Sunday, October 13.
New low-energy traffic lights will also be installed, replacing the existing ones.
The work will be done in stages to minimise disruption to motorists.
Between now and Friday, September 27, Upgate and London Road will be closed to motorists. But Newmarket and South Street will be open as normal.
Between Monday, September 30 and Friday, October 11, Newmarket and South Street will be closed, with Upgate and London Road open. The junction will be fully closed on Saturday and Sunday, October 12 and 13 to finish off.