Problem with creating a lookup field in existing records

Hello!

I have two entities A and B, with many records in them.

entity A has 1:N relationship with entity B

Inside of entity B, I've created a lookup field that display the corresponding primary key of entity A ( It directs me to the corresponding record in entity A when i click on them)

Problem:

I want to create a lookup field in entity A that display the corresponding primary key of entity B. 

As there're already many record in entity A, how can I do this so that the lookup field I create in entity A can automatically match to the corresponding primary key of entity B in each record?

Thanks in advance!