AlfaSim/Virtavia F-104 skin modification

Hello,
I've downloaded the replacement texture .479r file for the original Luftwaffe/Italian Air Force F-104G in order to provide a USAF version.  However, when attempting to leave and save the modifications to such file, a dialog box comes up saying I do no have the correct permissions to make such a modification.  What would you recommend to overcome this difficulty?  Any help will be greatly appreciated.  Many thanks.