changing a field from one table to another

There are aspects of this database that are important and which I do not know about your data. But, with that said ...
You should be able to first add the field to the customer table (an empty field with no data).

Then copy the data from the field in the product table to the field in the customer table, using an update query.

Then, once you know the data is properly in the customer table, delete the field and its contents from the product table.

What you need in order to do this is a way to "line up" the lines of data coming from one table to the proper records in the destination table. I am having difficulty at the moment envisioning a field in a product table, with each entry in the appropriate record, and how this same field's data would align with records in a customer table. There must be some relationship between these tables and, if that is the case, there should be a ways to set up the query to copy the data over. Again, I am trying to see in my head the sorts of fields in a customer table and the sorts of fields I would have in a customer table. Thus it is hard to see how the data in this field is allocated to the other fields.

My head is spinning out of control with thoughts of how a field could exist in any way in these two tables.

I don't think I can try to explain this in detail without seeing the table structure of the customer and product tables. By structure, I mean seeing what fields are in each table. And what the primary key is in each.