Hi,
Try: 'Profiles'>scroll down to 'Advanced' (hidden at the bottom of the screen)>select>click on the 'Profile' you would like to change the SMS tune in>scroll down to 'SMS Text'>select>scroll down to 'Ring Tone'>select>scroll through the options to see which tone you want (the tone will play as you scroll over them)>select>then exit and save changes. Clear as mud?

Don't worry, sounds complicated but will become clear when you follow the above.
Kind Regards,
Greg Myers