Hello Husam,
this link will bring you to S-series specific firmware (provided entitlements agree)
https://extranet.extremenetworks.com/downloads/Pages/S.aspx
For the long story:
From
http://www.extremenetworks.com/ find the grey bar near the top of page.
From the bar at top of pane, select
support portal.
Then from the bottom left, select (in red)
Downloads for EOS etc...
Here you'll need to
enter creds.
From the left, select (in red)
Switching & Routing
Then select
S-series - this will bring you to firmware, release notes and other documentation.
Hope that helps,
Mike