Hello,
I'm using my Splunk.com username and password to log in. I've also tried my email and password with no luck even after reseting my password.
Are you seeing any errors? Can you please post the error screenshot?
Additional note, if you are upgrading the App/Add-on which is managed by deployer/deployment-server/cluster-master then Splunk will not allow UI upgrade. You will need to upgrade the App/Add-on from the right manager component.