Error while modifying .h or .cpp file

I am a newbie learning more by the day. Working on a project with health sensors that I procured at cooking-hacks.com. As part of connecting their ehealth and wifi shield to the Uno, they require me to modify the Rx buffer size specified at HardwareSerial.h
When I open the HardwareSerial.h file on notepad to make the adjustments, I am unable to save that file back? What am I doing wrong?

When I open the HardwareSerial.h file on notepad to make the adjustments, I am unable to save that file back? What am I doing wrong?

The folder it is in is probably within the Program Files folder if you are using Windows, and that being a system folder, has special properties and the file is probably marked as read only. Check its properties.