Security patch for Samsung Galaxy S9+

[civilizations]

My friend's S9+ stuck to December 2018 Security patch since he bought it. Whenever he checked it says "your software is up to date" please help me resolve this problem.  Thank you. 

[Nelly81]

which android is it running

[civilizations]

Android 9 

[MittuVichu]

Try installing latest firmware manually.

[civilizations]

How do I do it please? 

[MittuVichu]

Download firmware from Sammobile and flash it with Odin. You can find lot of tutorials on YouTube.