Hi Satyabir,
The steps to add or change the key user can be found in the help document in your system with the title "Define Key Users for Project Team – Configuration Guide"
In the document, it is mentioned to find an Activity "DEFINE ADMINISTRATORS FOR PROJECT TEAM" in Implementation Projects > Edit Activity List.
Depending on your system, it may also be found in Business Configuration > Overview > All Configuration Elements > Search for "DEFINE ADMINISTRATORS FOR PROJECT TEAM".
I hope this information helps!
Kind regards,
Douglas Kisinger Thompson
Support by SAP Cloud KM