Hi Delnang

I think you need to do in the following way to solve your problem

1.    Open form in design mode
2.    Go to Tool menu and Select ‘form Option’
3.    In form Option, go to ‘Advanced’ tab
4.    Select ‘Automatically Upgrade for existing forms’
5.    Click ok and save the form

Now you can make the changes that you want and save the form and publish it
to the old form library.

I hope this will help you.

Thank You