If I update from v0.21c to v0.22 I am in danger of replaceing ownclowd with nextclowd?
If it is… How can I keep my ownclowd?
the update v0.22 does not replace Owncloud with Nextcloud. You can see all changes to v0.22 in the changelog.
Unimportant side question: why do you want to stay with owncloud?
Thanks, how can I perform the update?
you will just connect to your box via ssh and put in this command (that is the same command you installed your box with 
)
curl -s https://mailinabox.email/setup.sh | sudo bash
you can find more information about upgrading your box here,
and even more information and tips in the Mail-in-a-Box Maintenance Guide.