How to update foriegn key ?

Hello Experts,
I want to update a key in a table which is related to the primary of parent table. I thought updating the primary key in the parent table would automatically update the key in the other table. But it is showing error , asking to declare the key of the child table . How can i do that?
More detailed:
The first table is UserInformation , it has the primary key ID. The second table is ContactInformation , it has the key UserID which i have related to ID of UserInformation. My motto of doing this was, i wanted to have a field in ContactInformation which should show that this Contact belongs to this User because UserInformation is a table for registered users and ContactInformation is a table for Contacts of those registered users.
What I have done is , I have related , graphically , the key UserID to ID using sql server studio express. I have a createcontact page for creating a contact for registered users. I haven't included the field U ...