First, you must choose which SMS app you want to use as the default on the London. It is only possible to set the notification ringtone for the app that is set as the default.
To choose the default SMS app:
> Wireless & Network
> Default SMS App
If you choose Messenger as the Default SMS app, please proceed as follows:
> touch Messenger
> select 3 dot settings button in the top right of screen, choose settings
> click on sound, choose your notification ringtone.
If you choose Messaging as the default SMS app, please proceed as follows:
> touch Messaging
> select 3 dot settings button on the lower right corner of the screen, choose settings
>scroll down, select sound and choose notification ringtone