If you want to change to 8888:
grep -R 'ExecStart=/usr/sbin/varnishd' /etc/
/etc/systemd/system/multi-user.target.wants/varnish.service:ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a :8888 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m
systemctl daemon-reload
systemctl restart varnish.service