Hi Ott,
Welcome to this Microsoft Project newsgroup
I guess you're looking at a Summary task - it will have a bold font. The Duration of a summary is the working Duration between the start of the first task to the finish of the last task that it is summarising. It is calculated by Project and thus it cannot be changed.
FAQs, companion products and other useful Project information can be seen at this web address:
http://project.mvps.org/faqs.htm
Hope this helps - please let us know how you get on
Mike Glen
MS Project MVP
See
http://tinyurl.com/2xbhc for my free Project Tutorials
How do I "unlock" a duration? The up and down arrows are locked.