See the "Configuring FlexConnect AP Upgrades (GUI)" section.
Step 6 From the AP Name drop-down list, click Add Master to add the primary access point.
You can manually assign primary access points in the FlexConnect group by selecting the access points.
https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-6/config-guide/b_cg86/configuring_flexconnect_ap_upgrades_for_flexconnect_aps.pdf