I have installed texlive 2015 but when i try to do tex -version the shell prints that the current version is TeX 3.1415926 (TeX Live 2013/Debian)
texlive 2015 has been installed under :/usr/local/texlive/2015
Remove the version being installed via the Ubuntu repositories:
sudo apt-get remove 'texlive*'
Edit the path environment in /etc/environment:
sudo nano /etc/environment
Add the path to the texlive installation, e.g. for a 32-bit installation:
PATH="/usr/local/texlive/2015/bin/i386-linux:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
or for a 64-bit installation:
PATH="/usr/local/texlive/2015/bin/x68_64-linux:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
Replace /usr/local/texlive/2015/bin/i386-linux with the path in your system.