how can I update the field of the record on TN3270 screen? 
Can I use "putfield"? It works on adding a new record,but 
not updating a record. Thank you!

Yes, you can use TEACTION PutField for an update--the way 
that Screensurfer interacts with the TN3270 screen is 
identical, whether the application behind the screen is 
performing an update or add.

What might be different, however, is that frequently some 
fields that are set-up as input fields for an ADD are not 
available (they are protected) when performing an update.  
So, maybe one or two of your fields is not available by the 
application for update and that is causing a problem in 
your script...