You can use the OnSamlAssertionCreated event to access and modify the SAML assertion before it's sent to the partner SP.
_samlIdentityProvider.Events.OnSamlAssertionCreated += (httpContext, samlAssertion) =>
samlAssertion.Issuer = new Issuer()
Name = "name goes here"
await _samlIdentityProvider.InitiateSsoAsync(partnerName, userName, attributes, relayState);
There's also an OnSamlResponseCreated event if you wish to change the issuer field in the SAML response.
However, please note that we generally don't recommend changing the issuer as this is a static value uniquely identifying the identity or service provider.