> I have some idea with your process now ,but am I need to define the form b4 call the form replace ?
> If so ,can you tell me how ?

You said that you called REPLACE from your existing form MainForm, presumably with a line of code like:

DO FORM REPLACE

Replace as modified now accepts a parameter, the name of the object it's going to update, so from within MainForm:

DO FORM REPLACE WITH thisform

passes the object reference for the current form as a parameter.