Hello Milind,
You can actually change these fields. If you are in version 11x
1. Go to Settings -> Configuration -> Custom Fields
2. Click on Templates tab
3. Find the template that you are using for the specific department you are wanting to change (if this has yet to be configured it may just be Default Ticket Template)
4. Click on the template
5. Click on Field Definitions tab
6. Use the arrows under the "move" field to move them up and down, delete them or whatever you wish to do
7. Save your changes.
Hope this helps!
Note: You may be able to do it in 10x too in a similar way. Let me know if you need additional help.