To change you ringtone for text messages (or calls,voicemail etc.)

here’s how

go to the menu by pressing the middle button

Go to where it says settings

go to Ring style

switch it to which ever style you want (vibrate and ring, loud ring ect.)

under it it should say the style you picked and detail ex.(vibrate and ring detail)

click on it

there should be a line to change ringtones

line 1 (when someone calls you)
Text Msgs (when someone sends you a text message)
Voicemail (when someone leaves you a voicemail message)
Alarm (you ringtone for the alarm)