How to update related data programmatically

Hi guys,

I've problem in updating related data field programmatically?
Anybody faced the same problem before?
I was able to create and delete relation by using CreateRelation() and DeleteRelation() methods but no luck in editing.
There are 2 content types Eg: content type A and content type B.
content A has related data field which reference content type B.
What I'm trying to achieve is to change the relation in dynamic item in our customized Edit page.

Would be really appreciate for any help.

Thank you.