Hi Jean Carlo,
Thanks for trying out Qdabra's DBXL.
Have you tried out the Getting Started tutorial?
http://www.infopathdev.com/files/folders/installation_and_setup/entry85815.aspx
We also have some free training available here:
http://www.infopathdev.com/files/folders/dbxl/entry78020.aspx
So now to your question: There's really nothing you need to do in the form to overwrite. If you are opening a document that has been submitted to DBXL, make a change and resubmit, the document will be overwritten. If the document has not been submitted to DBXL it won't contain the DBXL Processing Instructions and DBXL won't recognize it as an existing document that it needs to overwrite.
Anyway, the Getting Started guide and the training modules show off the step by step. Let us know if you have any questions.
Thanks!