The following is right out of the API Help. The code assumes that the project is already in the lsit of available projects. If it is not then you will need to add it with the DesignProjects.AddExisting() method.
Public Sub SetActiveProject()
' Check to make sure a document isn't open.
If ThisApplication.Documents.Count > 0 Then
MsgBox "All documents must be closed before changing the project."
Exit Sub
End If
' Set a reference to the DesignProjectManager object.
Dim oDesignProjectMgr As DesignProjectManager
Set oDesignProjectMgr = ThisApplication.DesignProjectManager
' Show the current project.
Debug.Print "Old active project: " & oDesignProjectMgr.ActiveDesignProject.FullFileName
' Get the project to activate
' This assumes that "C:\Temp\MyProject.ipj" exists.
Dim oProject As DesignProject
Set oProject = oDesignProjectMgr.DesignProjects.ItemByName("C:\temp\MyProject.ipj")
' Activate the project
oProject.Activate
' Show the current project after making the project change.
Debug.Print "New active project: " & oDesignProjectMgr.ActiveDesignProject.FullFileName
End Sub