When you download Splunk Enterprise you should have set up an admin account once Splunk starts up. This account is used for conducting administrative duties for you Splunk Enterprise deployment even when it is in trial period.
You will need to login as the admin account and then give your user the admin role in Settings -> Users -> 'Your_user' -> Edit -> Assign Roles.
If you're looking only to change your user preferences I don't see why you wouldn't be able to do so. What exactly are you looking to change? You can certainly click on your user name on the top of the Splunk screen and then on Account Settings or Preferences depending on what you're looking for.