First, create file /etc/pki/tls/certs/YOURWEBSITE.COM.cert and paste the content in cert.pem you got from letsencrypt
after that login to your CWP admin > Apache setting > SSL cert manager
after that Install certificate using the form, select your cert file you created above, select your user and input domain
click install ssl
After installing, change the content below:
KEY : change with content from privkey.pem
CSR : change with content from cert.pem
BUNDLE : change with content from chain.pem
DONE