$ add-apt-repository -y ppa:jason.grammenos.agility/php
$ apt-get update
$ apt-cache policy php7.2
php7.2:
  Installed: (none)
  Candidate: 7.2.15-1+ubuntu16.04.1+deb.sury.org+1
  Version table:
     7.2.15-1+ubuntu16.04.1+deb.sury.org+1 500
        500 http://ppa.launchpad.net/jason.grammenos.agility/php/ubuntu xenial/main amd64 Packages
$ apt-get install -y php7.2

Then you need to disable old version and enable new one:

# Disable php 7.0
$ sudo a2dismod php7.0

# Enable php 7.2
$ sudo a2enmod php7.2

# Restart apache2
$ sudo systemctl restart apache2