Hi Omkar,
Just to clarify, the only error you see is the browser confirmation dialogue that says, “The update script failed without returning an error. Please contact tech-support,” correct? If that’s the case, this appears to be an AJAX error, so my recommendation is to take AJAX out of the loop here. After restoring your backup and ensuring that you wipe your database before importing the database backup, please try the upgrade again with JavaScript disabled on your browser.
Also, as a precaution against some strangeness we’ve seen with using Firefox to upgrade EE, make sure you’re using Safari or Chrome instead.
If you get any errors this time around, document them here and we’ll go from there.