To replace a project team member you must first make sure that the replacement resource has already been added to the project team.

The function pointed at the situation, when a planned Resource should be changed during Project Planning (stand-by).

Edit the Project description.

Click on the [Actions] menu and select "Rename a resource".

In the "Resource replacement" dialog box that opens, select the resource to be replaced and the replacement resource:

Once the replacement resource has been selected, all assignments made to the replaced resource will be re-assigned to the replacement resource.