Good morning, everyone,
I made an installation everything offers quickbox it works great!
I wanted to install nextcloud but I have the following error:
This version of Nextcloud requires at least PHP 7.1
You are currently running 7.0.33-7+ubuntu16.04.1+deb.sury.org+1. Please update your PHP version.
I wanted to know if you have a procedure to upgrade to 7.1 or even better to 7.3? 
Thank you for your help.
The following link should do what you require, but just got to check what and how it will effect all the various PHP modules that are used in the install of QB, but it shouldn’t in theory cause to much of an issue, but if possible make sure you have a current and up to date backup of your ubuntu install before preceding with the upgrade.
https://ayesh.me/Ubuntu-PHP-7.2
My install is in a VMware VM so I just need to create a snapshot of the OS before going ahead with any changes. It is also a wise idea to have a VM install the mirrors what you are using (doesn’t need the torrent downloads) so that you can then use that as testbed for any changes, and to see if and how they effect the “Live” server that you are using.
Hope this helps.