[deanhills]

I'd say it is quite simple. Check your mybb images folder to locate the images of the icons that are already there, and then replace them with the new icons by using FTP. Make sure not to change the names of the icons. Just copy the icons with the ones you prefer and call them the same name so they overwrite the icons that are already there. You'll then have to wait a while and be patient as in my experience sometimes it takes a little time for the change to take effect.