If you use bandsteering on any second gen device (UAP-AC-Lite/LR/Pro/EDU/M/M-PRO/IW/IW-Pro), then you will need to disable bandsteering, reboot the device, and then run the upgrade. You can re-enable bandsteering once the upgrade is complete. This is a one time process due to a bug introduced in 3.9.11, and earlier firmware versions are not affected by this.
Links are all HTTPS. This assumes your device(s) are currently running at least 3.7.51, and that the device's date is properly set (i.e. it can reach a NTP server). If you encounter issues with the upgrade, or your device isn't on a minimum supported firmware, then you can change the link to HTTP before performing the upgrade.