I will describe the ZyXEL MES-3528 switch firmware update procedure:
1) Download the new firmware version from the site
At the time of this writing, I was flashing with version 3.90 (BHR.5) 20151110. Download the downloaded archive.
2) Open the switch’s web interface by typing its IP address in the browser (standard user is admin, password is 1234), select “Management” – “Maintenance” in the menu on the left, then click “Firmware Upgrade Click Here” and click “File Path” “Browse” and select the firmware file previously downloaded and unpacked from the archive, tick “Rebooting” so that the switch reboots and starts with the new firmware, otherwise it can be rebooted later, and before that it will work on the old one.
Click “Upgrade” to start the upgrade process and wait.