How do I install the latest update firmware V5.21(AASZ.3)CO after download to the PC

If download file is zip file, you need to unzip it.
Then, go to Control Panel > System > FW Upgrade > Manual Firmware Upgrade, select 521AASZ3C0.bin and tap Upgrade.

Or you can refer to this FAQ.
https://homeforum.zyxel.com/discussion/3370/faq-upgrading-latest-nas-remote-code-execution-vulnerability-firmware#latest