There is no in-place upgrade between MariaDB major versions; it is too risky and too irreversible. To upgrade, create a new process and migrate your data from the old process to the new process. This approach removes the risk of an upgrade failure, and ensures you can "roll back" and continue uninterrupted if the upgrade fails.
The recommended way to migrate data is from the ssh command line, but it can also be done via phpMyAdmin if you prefer. To do this from the command line, export the data from the old MySQL process:
YourPrompt> mysqldump --user=exampleuser --host=exampleold.db -p --lock-all-tables --databases exampledb1 exampledb2 exampledb3 >migrate.sql
(Replace exampledb1 through exampledb3 with the names of all of your databases in the process. Do not include "mysql" or "information_schema" in the list.)
Load your data into the new process:
YourPrompt> mysql --user=exampleuser --host=examplenew.db -p <migrate.sql
(Replace exampleuser with your MySQL administrator username. Both mysqldump and mysql will prompt you for the password on the next line.)
After the data is copied over, you can perform any checks or updates you wish, like updating table types to take advantage of new features. You'll also need to create users in the new process for your web site(s). (What a great time to change/upgrade the passwords!)
When you are ready, update your site(s) to use the new hostname (and, hopefully, password). Save the old values so that if something doesn't work, you can switch it back.
Once you have confirmed that everything is working as you expect, delete the old process. You may want to put this off for a week or so to see if you missed anything, but make a note to do it so you don't keep paying for the old process indefinitely.