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