Hi Rishi,
Please follow the steps to resolve this issue and to make the header_jscripts.js editable :
1. go to the transaction SPAU
2. Enter the Package Name : CRM_IC_CMP_FRAME and press Execute
3. You will get a list of notes
4. Now what you need to do is right click on these notes and do a "reset"
Please click on reset all for all the notes.
After following all these steps, your header_jscripts.js will become editable. 
Cheers,
Devashish