By editing a field on the form, you are probably trying to change an entry in the table on the 'many' side of a 'many to one' relationship, which cannot be done in Access (neither can you Delete). You many have to think about creating a result table from your two tables, base the form on this, change the form, and hence the result table, and then write an Update query to update the table on the 'many' side.
How do you eat an elephant? Break it up into small chunks. Unfortunately, Access contains many elephants.