Just with one command and everything will be done for you...
Remind: Your machine will restart after executing this command
sudo -i && cp /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt.ORIG && curl -k "https://letsencrypt.org/certs/isrgrootx1.pem" >> /etc/ssl/certs/ca-certificates.crt && reboot