How to upgrade to the latest release php release?

By not affecting other software, you mean not upgrading them?

sudo apt-get upgrade

// to see new version number
sudo apt-get -s install php5

// to upgrade it
sudo apt-get install php5

If it doesn't show the new version, it may be a version update so Ubuntu or other distros won't update it without doing

sudo apt-get dist-upgrade

Checking upgrade policy

sudo apt-cache policy php5

I believe Forge is still using the Ondrej/php5-php5.6 PPA to get php 5.6 on Ubuntu 14.04. Looks like it has php5.6.6 built for Trusty, which means just @bashy said - sudo apt-get install php5 (install will also upgrade).

That may affect other software, so check what it will install/update before letting it through! (in general, it'll be safe to upgrade, however).

Here's what I see on my server (not forge, but very close match):

vagrant@vagrant-ubuntu-trusty-64:~$ sudo apt-cache policy php5
php5:
  Installed: (none)
  Candidate: 5.6.6+dfsg-1+deb.sury.org~trusty+1
  Version table:
     5.6.6+dfsg-1+deb.sury.org~trusty+1 0
        500 http://ppa.launchpad.net/ondrej/php5-5.6/ubuntu/ trusty/main amd64 Packages
     5.5.9+dfsg-1ubuntu4.6 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
     5.5.9+dfsg-1ubuntu4 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

Note that it says it's not installed. If you see the try, try php5-common:

vagrant@vagrant-ubuntu-trusty-64:~$ sudo apt-cache policy php5-common
php5-common:
  Installed: 5.6.6+dfsg-1+deb.sury.org~trusty+1
  Candidate: 5.6.6+dfsg-1+deb.sury.org~trusty+1
  Version table:
 *** 5.6.6+dfsg-1+deb.sury.org~trusty+1 0
        500 http://ppa.launchpad.net/ondrej/php5-5.6/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status
     5.5.9+dfsg-1ubuntu4.6 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
     5.5.9+dfsg-1ubuntu4 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

The *** means that's the installed version.

Yeah may vary depending on how it was setup. Can always run the --only-upgrade flag if you don't want to upgrade any packages not already installed.

sudo apt-get install --only-upgrade <packagename>

https://murze.be/2016/04/upgrade-php-5-6-7-forge-provisioned-server/

@insign That's his own website :D