If you need to view them again to accept the certificate you can go to vCenter via URL and download then install it.
You must remove the certificate from the Browser you are using. After removing it you will be asked again.
Go to Tools-->Options-->Advanced-->Encryption. Click on [View Certificates]. Select the "Servers" tab. Select the exception to delete. Click [Delete].
If you use other types of brousers and you don't find the indications write to me as well.