Hi Sima,
Thank you for reaching out.
You will be able to update Work on tasks (child items only). The work column is locked at the project/milestone level as the work value is a roll up from subitems (tasks).
For further clarification, please join our Q&A webinars (see the link-> http://www.clarizen.com/resources/online-demo.html#second ).These are great resources for free consulting with our Customer Success Managers and visual answers to your questions. One of our experts will be able to discuss your specific business requirements and suggest best solution accordingly.
All the best,
Diana