After upgrading from Splunk 6.1.3 to 6.3.1, why is my license showing as expired/not recognized, but it expires in September 2017?

I proceeded to Splunk version upgrade from 6.1.3 to 6.3.1 and I am having a problem that license has expired or is not recognized. The license is 2GB and expires in September 2017. My server is running on Windows Server 2008 R2. What can be the solution to my problem?