How does the Project Manager or the Contractor notify of an early warning?

An Early Warning is a notification of a matter that could affect the price, the programme or the performance of the works.
There is no reply per se required to an early warning notification but the parties may require the other to attend an early warning meeting (also called risk reduction meeting) if the matter required the parties’ attention immediately.
The Contractor can be sanctioned for not raising an early warning that he could have raised.