Let's say something like this is being returned.

Setting tune.ssl.default-dh-param to 1024 by default

The easy fix is to add tune.ssl.default-dh-param 2048 to the global section of your /etc/haproxy/haproxy.cfg file.

global
    log         127.0.0.1 local2 debug
    chroot      /var/lib/haproxy
    pidfile     /var/run/haproxy.pid
    maxconn     4000
    user        haproxy
    group       haproxy
    daemon
    stats socket /var/lib/haproxy/stats
    tune.ssl.default-dh-param 2048