@sharanya_ps,
Since you are using older version of the product so, you might need to update your existing code when upgrading to latest APIs set.
For your information, over the years, we included many enhancements (regarding existing features) with other fixes and supported some new features. We have made the product more robust and feature rich APIs set now. You got to update your code (in accordance with latest APIs set) as we made some changes with existing APIs. We have reorganized the APIs structure and moved certain APIs to their corresponding namespaces since the release of v5.0.0.
We recommend you to kindly see and browse Aspose.Cells for .NET Docs for your reference:
http://www.aspose.com/docs/display/cellsnet
If you still find the issue while upgrading to latest Aspose.Cells version regarding code changes, do let us know with details and samples, we will check and help you through.