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