Select the member you want to replace
Project> 
“Settings” icon in the top right corner of the window. Go to the “Collaboration on project” section.
Select the member to be replaced by checking the box on its left.
Choose the member you want to assign to the tasks of the selected member
Click on the “Replace” button 
located above the list of project members.
A pop-up window appears, you can then choose:
- Which member to replace the initial member with
- A start date for the replacement. All tasks scheduled from this date will be affected by the change of assignment.
Validate the replacement and note, in the Who view of the project, that the change of member has been carried out on all the planned tasks, from the chosen date.