NC 20.0.5 shows warning on MariaDB 10.2 or higher. How to upgrade?

Hello All,

I am upgraded to NC 20.0.5 on Raspian Stretch (v9) and I got the following warning:

  • MariaDB version “10.1.38-MariaDB-0+deb9u1” is used. Nextcloud 21 will no longer support this version and requires MariaDB 10.2 or higher.

Could anyone suggest me how to upgrade MariaDB to release 10.2 without lose data and configuration currently in place?

Thank you in advance for your thoughts.