I am not sure what kind of unknown issue I am going to encounter with the TKL Nextcloud, but so far I can upgrade the Nextcloud instance without any issues. Upgrading TKL Nextcloud or any Nextcloud instance is pretty straight forward.
Navigate to Settings > Administration > Overview. Click on the Open updater button to start the upgrade.
Click on Start update as shown in Figure 2. If the update fails, click Retry. If retrying keep failing, make sure that the user www-data is the owner of the /var/www/nextcloud.
Once all the steps are complete, click on the No (for usage of the web based updater) button as shown in Figure 3. Afterwards click on Go back to your nextcloud instance to finish the update
Click on Start update on the bottom of the screen as shown in Figure 4
That’s about it. If you have to more upgrades to do, just repeat the process for each upgrades.