Vista - [LINQ TO SQL] Probleme for update a child element

(sorry for my english i'm french) :

3 tables:
[APT]
#AptId as integer
AptRef as Integer
NbPeople as integer

[APTREF]
#AptRef as integer
AptId as integer

[Request]
#RequestId as integer
RequestAptRef as integer

A relation is here : [Apt].AptRef <=> [AptRef].Aptid

Problem :

In my code :
MyRequest = (from MesRequest in dtct.Request _
where MesRequest.id=1 _
select MesRequest).single
For default, linq provided me MyRequest.AptRef as AptRef
When i want to update this :
MyRequest.RequestAptRef=Value (integer)
or MyRequest.AptRef= (from MesAptRef in dtct.aptref where MesAptRef.aptid=1
select MesAptRef).single
Error message ...

How to update this element?