How do I edit an info.plist file inside of a kext file?
All files under /System/Library is protected by SIP. So in El Capitan you can't do that without disabling it. 

Boot into Recovery HD, open Terminal under Utilities menu, and type:

csrutil disable


Restart you computer. Navigate to the kext file, right click Show Package Content. Now go to Contents then you can find the Info.plist file. You might needs to change permission of the file before you can edit it. After you're done, boot into recovery again. Open Terminal and type:

csrutil enable


Then reboot and you're done. 

But in later versions of macOS you may also have SIP enabled and only disable the kext-sign requirement, which may be a bit more secure than having it disabled entirely:

csrutil enable --without kext


Note that your changes might be reverted at a macOS update.