Deleting a foreign key relation then adding it again

Hello,

I'm currently updating data in an oracle database using sql developer.I have 2 tables that have a foreign key relation, I tried to update both of them together but it didn't work so I dropped the foreign key relation and updated them.Now I'm trying to add the relation again but it fails because the table has data. How can I solve this problem?
Or if there is another way to update both tables I 'd highly appreciate it

Thanks