In Android, you can change the default ringtone for all incoming messages from the messaging settings screen:
Navigate to Settings > Notifications > Your Android OS notifications settings screen will open, where you can change the phone's ringtone.
How can I change a contact SMS ringtone?
You can also change the ringtone for a specific contact incoming SMS.
To do that, open Contacts+ > switch to the SMS tab > select the relevant contact thread > touch the 3 dots menu button on the top right > select "Individual SMS ringtone.