Version 5.0.21.18 should already be installed. What version do you have?
The process for setting an individualised notification sound (not really a ringtone) is fairly convoluted. So in case you do have that version installed I'll explain the process I went through to set an individualised sound.
I select the Messages app, then select the *Conversations* tab, then I *tap* the entry for my mate "Chasse". Tap the 3 dots on the top right-hand corner. Under "Conversation Settings" appears the sub-heading "Notifications". By default it is set to "Default". Tap on it, and select "Custom", then tap on the settings cog to the right which takes me to the "APP NOTIFICATIONS" page where I can set the notification sound. Samsung provides a truly dull selection of notification sounds, so I put my own ringtone mp4 file in the Notifications folder. That audio file will get appended to the list of notification sounds.
Actually, all I cared about was individualised ringtones for voice calls. Samsung gutted its Contacts app, so I purchased Drupe to fulfil that function.