Edit items in table model

[gyre]

Hi , I have a question...
I have a table model...working fine...
the only thing that makes me a bit angry is that everytime when I try to edit items there is no way I can add new data to the ones that it already contains it...when I try to edit data in an item the old value that is stored in it gets disappeared and I get empty item with blinking cursor...yes If i press Esc the old value gets displayed...
What I want is standard behaviour of table editor->> when I double click the item I want EDIT the value not INSERT NEW value in edited item...
Anyone idea ?
THanks !

my model implementation is in attachement