adddescription to user account in linux

I am root in my linux server and create a testuser account. but i forgot to add a description to the user account. how can i go back and add the descriptions to the account.

It just looks like this in the passwd file:
testuser:x:512:512::/home/testuser:/bin/bash

First you can switch to usr/sbin then run the following command:

$ usermod -c "your description" testuser