It is important to keep your device firmware up to date for stability and security reasons, for example, the WPA2 protocol was found to have a vulnerability which requires a firmware upgrade to address. Info about the key reinstallation attack, or KRACK, can be read here.
Log into your Cisco WAP121 by typing its IP address into your browser address bar, and type your username and password.
Go to Administration, Upgrade Firmware:
Click Choose file, the latest firmware can be downloaded from the Cisco website.
Make sure it is the TAR file.
Once you have selected the file, click Upgrade.
You will see a warning before proceeding.
The firmware will upload:
And then install, do not power off the AP while this is going on – the AP will not be usable at the same time either.
This part takes a few moments, and then you will be prompted to log back into the device:
You can verify the upgrade is complete:
There is a video showing this process available here also.