Thanks, and you're not being blind. The underlying protocol (ACME) does have official support for certificate renewals, however, this has not yet been implemented in ACMESharp.
However, you can use a kludge that will provide effectively the same outcome. You just need to submit a new certificate request, and just reference the same Identifier (i.e. DNS name) that you used for your first certificate issuance. Once you have verified you are the owner of an Identifier, that verification is good for about a year (after that you'll need to re-verify your ownership).
You can see more information at
this issue. I've also updated the
FAQ to address this question.