When I originally setup my ConfigMgr environment setting up my OSD PXE certificaates was kind of a set and forget process so when my deployment team started getting a prompt about an expiring cert I thought "Oh ya I forgot about those."
In order to update your PXE certifcate you need to locate the site system that hosts the PXE Service Point where you want to update the certificate, double click the PSP role, on the database tab select create a self-signed PXE certificate and specify an appropriate date range and then click apply. As soon as you create your new certificate your old one will be blocked. It's just that simple but it's one of those tasks that you probably don't do that frequently.
If you are running in Native mode will have to import your new certificate from your root certificate authority. (CA)
