Modifications to wp plugin deleted after a few days

I have a small wordpress (4.3.2) website and for the contact form I have used the free version of the Contact Bank plugin.

I have modified the one of the plugin's files (contact_bank_forms.php) located in /wp-content/plugins/contact-bank/frontend_views/ and everything looked good.

But after a few days I have noticed that every modification i made to this file was lost and realised that it went back to the original code - like I have not modified anything.

I reuploaded my modifications but it seems that every few days it goes back to the original one.

Can anyone please help me with this problem?

via Chebli Mohamed