Via Terminal

sudo nano /etc/sysctl.conf

…. add at the bottom the following lines

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

… save and quit

Ctrl o
Ctrl x

… and run

sudo sysctl -p

An another way is to edit /etc/default/grub file editing the line as

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=True"

After editing run

sudo update-grub