Log into your server via your FTP Client (I recommend WinSCP it can be obtained for free and works great, others have their personal preferences also)
Navigat to the file path that is requesting a 666 permission.
Right click on the file and select "Properties"
You will see a box appear with check marks in the first 3 columns and none in the 4th. These are your CHMOD permissions. You will also note it says directly below all the boxes with checkmarks another box that says 0777 in it.
Change that 0777 to 0666 and then save. This will change the file permissions.
If you have multiple files in the same folder you can hold the CTRL button and click on them until they are all highlighted and then right click, properties and change the permissions on them all at once if they are all going to have the same permission.