How to Upgrade from nextcloud 15 stretch to nextcloud 16 buster

I have a raspberry pi 3+ with nextcloud 15.0.11 on stretch, php 7.0.33.
I would like to update to nextcloud 16 on buster php7.3
What is the procedure ?
Or shall I make a backup of nextcloud, and make a new fresh install of buster?
Thanks for your advice.

No one answer? No idea?

Do a backup!

I’d first update raspbian to buster and make sure that everything works. Then upgrade to NC 16. Don’t do both at the same time. This is if you have installed the system yourself and don’t use pre-installed images (nextcloud-pi, docker, …), there might be better solutions.

Okay. Thanks I’ll give a try.