If only copying network devices, network device groups, internal users, and internal groups, then we may use CSV export these items from ISE 2.1, adjust the columns, and then import them to ISE 2.6. Otherwise, we may take a configuration backup from ISE 2.1 and restore it to ISE 2.6.
From ISE 2.1 to 2.6, we will get the new policy UI so please take a look at the UI walkthrough at
What's New in ISE 2.3? - Cisco Community