You can go under the wlan ssid-profile and select the rates you want enable under that ssid
(controller) (SSID Profile "TEST-SSID-PROFILE-B") # g-tx-rates ?
1 1 Mbps
11 11 Mbps
12 12 Mbps
18 18 Mbps
2 2 Mbps
24 24 Mbps
36 36 Mbps
48 48 Mbps
5 5 Mbps
54 54 Mbps
6 6 Mbps
9 9 Mbps
You can do it from the GUI : Configuration > All Profiles > SSID > SSID Profile