You can do a couple of things (you can't manually force it from the controller perspective):
- Remove lower data rates under the SSID profile
- Set a minimum local probe threshold to 25 (if you have hidden SSID otherwise this won't work all the time)
- do a show ap arm rf-summary <apname> or show ap arm history <apname> to determine the current power levels on the AP (maybe the minimum power are too high)
- Update the drivers and if you are using windows some wireless cards allow to manage the roaming settings
Note: remember that is up to the wireless card to roam so depending what algorithm is use to determine the SNR/RSSI values that the device can see from the APs