You can use a site like this SSL Checker
An auto-renew may have been setup by Certbot depending on the options you chose. It just might not be working.
You can test a renew with this command. It will try to renew your cert against the Let's Encrypt test system and will not overwrite any cert on your system.
sudo certbot renew --dry-run
If your cert has less than 30 days before expiry then we'll need answers to the questions from the form you were shown when posting this topic. So, let us know what the above shows and we can go from there.
Especially when you admit to not being very experienced it is important we know at least your domain name to provide quality prompt advice.