Anyone can update Entropedia, or create new entries, even if not a registered user there.
If your former soc name was listed there, you can just edit the entry as Rowan has described. If it's not on the list at all though, you can add a new entry.
To add a society to Entropedia, look for
Avatars & Societies on the left side menu (in the 'People' menu group). Selecting this will bring up the Societies chart.
Then select the Add Item tab at the top of the page, and add the information you want displayed.
Note that the Leader and Founder columns are drop-down lists, so if you want these names to show for your society and they're not already listed, you will need to add the names to the
Avatars table.