Thanks for your help.
To help the next person who asks the question, here is the complete procedure:
SSH console:
Download the latest version from https://github.com/mattermost/focalboard/releases
On this page, locate the one that bears the tag “latest”
As I write these lines it is v7.11.4
Open your ssh console and go to the Mattermost installation folder
If you don't know where it is, follow the "Find the Mattermost installation folder" procedure at the bottom of this post
Then move to ./data/plugins
At home: /home/yellowtent/appsdata/aaaaaaaa-bbbb-cccc-dddd-3be7e9371bb7/data/plugins
Type the command: wget https://github.com/mattermost/focalboard/releases/download/v7.11.4/mattermost-plugin-focalboard.tar.gz
Then: tar -xvzf mattermost-plugin-focalboard.tar.gz
Finally: rm mattermost-plugin-focalboard.tar.gz
Restart (if you don't know how to do this, follow the "Restarting Mattermost" procedure)
In Mattermost
Click on the 9 squares by composing another at the top left of Mattermost then “System Console”
In "PLUGINS" > "Mattermost Boards, activate the plugin
Return to your "Channels"
Boards now appears 
“Find the Mattermost installation folder” procedure:
Go to mattermost settings (if you don't know how to do it, follow the procedure "Find Mattermost settings"
Left click in “Storage”
There you will find the Mattermost storage folder.
At home: /home/yellowtent/appsdata/aaaaaaaa-bbbb-cccc-dddd-3be7e9371bb7/data
“Restarting Mattermost” procedure
Go to Mattermost settings (if you don't know how to do this, follow the "Find Mattermost settings" procedure
Click in the left menu on “Repair”
Then “Restart App”
“Find Mattermost Settings” Procedure
In Cloudron, in My Apps, click on the small toothed wheel that appears at the top right when hovering over the Mattermost apps
You arrive on the settings page