This article indicates the upgrade path for FortiAP-U models.
Scope
FortiAP v6.X and v7.X.
Solution
The following are Product Offerings for FortiAP-U:
The Firmware Upgrade path to follow for these models is as follows:
v6.0-build0028 > v6.0-build0080 > v6.2-build0220 > v6.2.1-build0237 > Then to Special Build v6.2.2-build4001 > v6.2.2-build0267 > v7.xx.
The special build is also available at support.fortinet.com