Nextcloud 12 to Nextcloud 15 or 16 upgrade

Hello all,
We have a running Nextcloud 12 instlled in Ubuntu 16 which we now want to upgrade to either nextcloud 15 or 16. Now we need help with the upgrade steps.
Please anyone with a good upgrade step by step instruction should kindly help us with this so that we can sucessfully carry out this upgrade.


upgrade steps?
well first and foremost: backup. and backup. and last but not least backup.
and then try the built-in updater. don’t try to skip major releases unless updater suggests so.

good luck