What system are you using?
I reiterate once again. THERE WILL BE NO UPGRADE OF WINDOWS RT 8.1 TO WINDOWS10.
At best they stated that SOME of the features that have been incorporated into Windows 10 will be released for Windows RT but what and when has never been detailed, you may find that all that would be has been.
Article 18581 has the following
"Microsoft will make software updates, including security updates, available for Windows RT. More information about the Windows RT lifecycle policy will be communicated as it becomes available." The article was reviewed May 2017
Given that officially support for the original Surface tablet hardware was due to end April 2017 and the two streams are closely integrated we can't expect too much.