From your issue, any gitea command (like gitea admin change-password --username myusername --password asecurenewpassword) ends up with:
gitea: command not found
If you installed from binary, you will note that the $PATH was not modified, and gitea was called directly from its installation folder.
./gitea web
So you can do the same for changing the password:
cd /path/to/gitea
./gitea admin change-password --username myusername --password asecurenewpassword
Note that Robert Ranjan adds in the comments:
/path/to/gitea is gitea's home path, where you find folder custom.
  In my case gitea's home is /var/lib/gitea. From this path, you should see file: custom/conf/app.ini which is expected by default.