I am not sure if you can use a Main-Form/Sub-Form scenario to manage cascading updates. I suspect you would have to create the correct master/child link properties of the Subform and also use the Relationship Wizard to define relationships and Referential integrity rules.
Having said that, if you are going to use VBA to update a field, you will likely bypass any intrinsic cascading tools. I do not use the Relationships Window in Access and I do not define Referential integrity rules in Access. I will depend on VBA to enforce most constraints and maintain referential integrity.
So, if you want to UPDATE many records, I would use an SQL statement and execute that statement. You can use VBA that would be similar to this.
Code:
Dim strSQL As String
Dim MyVariable as Long
MyVariable = Me.SomeField.Value
strSQL= "UPDATE tblName SET tblName.FieldToUpdate = 'Some New Value' WHERE ((tblName.PrimaryKeyName)=" & MyVariable & ");"
Currentdb.Execute strSQL