This wikiHow teaches you how to change the sound your device plays when you get a notification from Facebook Messenger.
Step

Step 1. Open Messenger
This app is marked by a blue speech bubble icon with a white lightning bolt which is usually displayed on the page/app drawer.
If you're not logged into your account yet, type in your Facebook login information at this point

Step 2. Touch the profile settings icon
It's a gray circle icon with a human image in the upper-right corner of the screen.

Step 3. Tap on Notifications and Sounds

Step 4. Slide the “Notifications and Sounds” switch to the on or “On” position
If the switch is already white (the “On” position), skip this step.

Step 5. Slide the “Sound” switch to the “On” position
If the switch is already blue (the “On” position), skip this step.

Step 6. Touch Notification Sound
This option is under the “Sound” switch.

Step 7. Choose a ringtone
When you touch a ringtone option on the list, you can hear a preview of it.

Step 8. Touch OK
After that, the selection will be saved. Now when you receive a notification on Facebook Messenger, the device will play the sound you selected.