Help with relationships between tables - changing connecting field
I have a really limited understanding of the complicated workings of Filemaker, but as a small nonprofit we can't afford professional help so I must bungle my way through. I apologize if this question is not well-worded or very simple.
Right now we have a database with two related tables - Contacts and Contributions. The relationship is currently based on the "Contact Name" field, which has the significant problem of dis-associating Contacts and their multiple Contributions if the name is changed at all. Each contact also has a unique Contact ID number, that was assigned belatedly, after the database was constructed.
My question is, how can I add the Contact ID number to the Contributions table, thus enabling me to change the relationship to being based on the number rather than the name? Right now the ID is displayed on the page, but when I change the relationship it no longer knows how to find that data.