Here are the instructions. The link to the actual certificate generator is above. (It’s in these instructions as well.)
The page where you upload the new certificates is from cpanel, in the Security section, click on SSL/TLS. On the right side of that page is a link Manage SSL Sites. Click that.
At the bottom of that page is a section called “Install an SSL Website.” Select the gregoryberry.net domain from the dropdown.
Now, on the Let’s Encrypt page where you generated the new certificates, copy the first box (CRT+CA Bundle). Paste that into the top box on the cpanel page. That text block actually has TWO certificates. You have to find the beginning of the second certificate, CUT it from this box and paste it into the bottom box (CABUNDLE).
Then copy the second box on the Let’s Encrypt page (Private Key) and paste it into the second box on the cpanel page (Private Key (KEY)).
That’s it! Hit Install and you’re done. Test it to make sure it’s been updated!