Go to Organization->Firmware upgrades
Then the "All Networks" tab
It will give you a list of networks and firmware status, select the network(s) and then click the "Schedule upgrades" button.
(Or at network level, go to the Network-wide->General page for the network, and you can also do it there.
If you don't get the option to upgrade, check that your AP(s) are supported, from release notes...
This version, and subsequent MR 27.x versions, are only supported on the 802.11ac Wave 2 and 802.11ax MR product portfolio. 802.11ac Wave 1 and older products will not upgrade, should they be in networks configured for MR 27.x versions, and thus the new features will not be available and the bug fixes listed will not apply.
If your APs are supported, but no upgrade is enabled, contact support as maybe they need to enable the beta on your network(s).