This short guide will show you how to easily change your profile cover image. before you do this, I would recommend you first prepare the cover image by following this guide
Profile Cover Image
This will ensure your image fits perfectly and looks great.

Click Profile from the My Quick Links menu

This is where the cover photo appears

Click 'Change Cover Image'

Choose an image on your device

Click 'open' or 'save' (depending on your device) and the image is uploaded and changed

Thats it! really simple but makes your profile more interesting. have fun!