Do you mean the record source has a new field and you must add it to your form? Just go to design view, click on the field list button, then drag the new field onto your form.
Or do you mean you have to add a new field to the record source? In that case, if it's a table, and the new field to the table design and do the above. If your source is a query, open up the query to find the tables creating the recordset and then pick the table you need to add the field to.