I don't have a lot of experience with the attachment data type, but to update these attachments through code, you will have to use the Recordset2 type of Recordset object (the recordset type added to access to deal with these new data types), and the Field2 type. Attachments are a type of multi-value field, and are navigated by referencing the value of the attachment field as second recordset object that you can use to iterate over the different attachments. Here is a brief example to give you an idea, naturally useful code will be much more involved (not tested, so test thoroughly first):
Code:
dim rst1 as DAO.Recordset2, rst2 as DAO.Recordset2, fld as DAO.Field2, db as DAO.Database
Set db = CurrentDb
Set rst = db.OpenRecordset("TableWithAttachmentsHere")
If not rst.EOF and not rst.BOF then
Set fld = rst("AttachmentField")
Set rst2 = fld.Value
Do While Not rst2.EOF
'Some Code here
rst2.MoveNext
Loop
End If
There are functions part of Field2 that are used to change this file data. In the example, to add a new file, you would use (after AddNew), rst2("FileData").LoadFromFile yourPathVariable. To save the file to disk, rst2("FileData").SaveToFile yourPathVariable. To delete the file (which it seems you want to do first), run delete on the record you want to delete (simply rst2.Delete).