You can't delete the appliance certificate. You can only regenerate the self-signed certificate, create a CSR or Import a CA issued certificate. There are PowerShell Cmdlets that will assist you with this (take a look at New-HPOVApplianceSelfSignedCertificate help and then Related Links for related Cmdlets), if you would prefer not to natively use the REST API (the POSH Cmdlets do natively interact with the REST API so you don't have to).
