Hey Venkat,
Follow the below steps to change duration display.
1. Open MPP
2. Click on File->Options
3. Click on "Advanced" -> scroll down to "Display options for this Projects"
4. Change the display type from the drop down values for Minutes, hours and Days. Click save, ok.
That's, you should be able to see the changes in the duration field.

Rajkumar Allepu (MCP) , | https://sharepointprojectserver.com ** Please mark it as answer if my answer would resolved your issue.