Hi @lelelele welcome to the LE community forum 
The document @MikeMcQ provided does a good job of explaining things.
But it mentions that OpenSSL 1.1 or greater is required.
Well... there has been a recent update to that; And now some older versions of OpenSSL can be patched to also work as well. For that, see: Old Let’s Encrypt Root Certificate Expiration and OpenSSL 1.0.2 - OpenSSL Blog
and also read through this post that includes some more technical details (like that one):
Production Chain Changes - API Announcements - Let's Encrypt Community Support (letsencrypt.org)