To delete SSL certficate in IIS,
1) Start Internet Information Service (IIS) Manager
2) Click on Server Name (server hostname)
3) On right side, click on Server Certificates button.
On next screen, you will get a list of all SSL certfificates. You can right click on any of the certficate and delete it.
For older version of IIS
1) Open MMC (Start > Run > MMC). Open the Certificates Snap-in.
2) Select Local Computer Account
3) Select Certificates > Personal
4) Find the certficate and delete.