Thanks for reply! You can not upgrade Standard SSL certificate to Wild card certificate. You need to purchase a separate Wild card certificate and then need to create a certificate binding.
Follow these steps for Create binding.
Check below document link for App Service certificate:
https://learn.microsoft.com/en-us/azure/app-service/configure-ssl-certificate#import-an-app-service-certificate
Let us know if further query on this or issue remains.