An adapted example form to run once per day at 1:26 a.m. would be
26 1 * * * cd /usr/local/letsencrypt && ./certbot-auto renew -q
It would be good for you to run this command yourself once from the command line to make sure that the renew currently works. Once you’ve seen it work (since, as you pointed out, your certificate is already due to be renewed), you can then put it in root’s crontab and be relatively confident that it will work automatically in the future.