Ravinder,
As an urgent solution, press Ctrl, Shift & Esc keys together. You can press Ctrl, keep it pressed, then press Shift, now keep both these pressed and finally press the Esc key. When you get the Task Manager window, release the keys.
Now, in Task Manager, if you don't see 'Processes' tag at left top, look at bottom left corner. You will see 'More Details'. Click on that.
Click on 'Processes'. You will see a list of running processes. Click on 'Names' at top. The list will be sorted on process names. Scroll to find your application name. Right click on it. You will get a small window with options. Click 'End Task' on it. Your application will be released.
Now, you must be able to overwrite the file.
By the way, you will have to find why the application was not released even after executing QUIT command.
Is the entry point into your application, ie the first code that gets executed, a PRG or a FORM?
Please come with more details. There are many well experienced people who can help you.
Rajesh