Download iTunes on your computer .Then connect iPhone via USB port on your computer.Open the "Applications" folder in "Finder." Double-click the "iTunes" icon, if you are using a Macintosh computer. Click "File" at the top of the menu. Select "Transfer " from the drop-down menu.Unplug the iPhone only after the "OK to Disconnect" message is displayed.
You can always use iTunes to transfer music or other media files to iPhone 6. Free apps like AirMore or AirDroid are also nice options. If you have trouble in using them, you can read the following guide.
It’s really easy. Install iTunes on your PC, then open the application. Drag and drop individual songs or entire folders of songs into the iTunes window and it will add them to your iTunes library. You can also go to iTunes on your PC, then select FILE>ADD TO LIBRARY, choose the file/folder, then click open.