I have simple query.

Suppose if I am working on a project which contains one form (frmStudent) which is linked to table tblStudent.
Suppose if I release this this application. And after some time I created a new form in my project (frmTeacher)
And again for this form I have created new table (tblTeacher) in my database.

Now my question is, while i had this software delivered earlier (when it had only one form (frmStudent)) my client had inserted some records in database. After I update my application and again released my application, what will happen to these past entries? because I dont want to lose any past work of my client.

It is your responsibility as developer to ensure your update is backward compatible and/or can import the old database to the new.

The simple update will be to update the project its self. Meaning you will create your project and publish/release it, the if you feel of adding new features you need to update the already created project in that way when you publish its will create a new version folder and should an end user who is using an older version download and install the app will only change/add the new added features and the project will still be as it was with the new features now.

To test simply create a project then install it on your computer and update your project and install it again(installing the new version) you will see. I do this a lot before I release my projects. At least in VB.NET