The intended way to renew is ./certbot-auto renew. It will check on the status of all installed certificates and will only try to renew if a certificate is less than 30 days from expiring. You can force it to renew sooner with ./certbot-auto renew --force-renew, but if you do this more than once per week (or if you have more than one certificate), you may be blocked by rate limits.
If ./certbot-auto renew is doing the right thing for you, we intend for you to automate it by making a crontab entry so that cron will run this command frequently. (The ./ at the beginning means that you have to change into a particular directory to run that, so the cd /that-directory && should be prefixed to the command in the crontab entry, or else you should specify the full explicit path to where you downloaded certbot-auto.)
A future version of Certbot will create these crontab entries for users automatically.