First step, only if you are behind a security/corporate proxy, skip otherwise.

As I am behind a proxy, (if not sure just skip) I should set my proxies

If you are just using synaptic (=Mint package manager) there is a menu setting for proxies, set them as in your browser. Easy done.

Adding ppa repositories from synaptic does not worked for me (even changing many config files...)

To manually configurate the proxies for system update your sloud edit or create a standard configuration file as (if no present  just create it):

As you will probable need to use apt-get commands you will need to edit a 'conf ' file (if not just create it)  as:

 /etc/apt/apt.conf.d/95proxies   or sudo pluma  /etc/apt/apt.conf.d/95proxies

Add the following lines (where proxy_name and port_number match your known local settings):

Acquire::http::proxy "http://proxy.mydomain.com:PORT_NUM/";

Acquire::ftp::proxy "http://proxy.mydomain.com:PORT_NUM/";

Acquire::https::proxy "http://proxy.mydomain.com:PORT_NUM/";

If you need to set the environment values for the proxies remember to do them BOTH in lowercase and UPPERcase. (Your life will be easier). See details in this searched help page: [page]

Once completed the proxy's settings -if required- follow with the real single command:

Adding the Libreoffice Stable ppa repository (as per version 4.3)

Current version information for ppa files is available at: https://launchpad.net/~libreoffice/+archive/ubuntu/ppa

In a terminal/shell window type (or simply copy and paste)

sudo -E apt-add-repository ppa:libreoffice/libreoffice-4-3      (edit -n-n as in current version)

Note: this -E flag is to export your environment (and proxy settings...) to sudo ...

Execute [Enter], you will be informed about version being installed (pe. 4.3), and then just confirm the installation. Done.

Then you only need to update your system as usual, as with:

• Automatically (system tray widget),
• or with package-manager (synaptic)
• or manually : 
  • sudo apt-get update     and followed with    sudo apt-get upgrade

Your libreoffice will be keep updated to the current stable version (as 4.3.x ...).

Future versions: You should check for new versions (4-4 4-5 ...) at the ppa site

Final note: This Libreoffice update will not generate a duplicate installation (avoiding the wasted space) as you may have did (as myself) by downloading/installing (*.deb files) from the official Libreoffice website. It will merge with your Linux Mint LTS update system. I just uninstalled my previously duplicated installation 'downloaded from Libreoffice .deb files' with the 'Synaptic Package Manager'. No issues.