It depends on the firmware. In ddwrt you can change it under Wireless-basic settings-wireless network name (SSID).

For HotspotWrt you can use these instructions: http://hotspotsystem.com/deskpro/kb_article.php?ref=1218-DLZV-3307

You can also set up Coova/HotspotWRT/ DDWRTfirmwares to accept commands remotely, then you can change the SSID from the control center. See this article on the topic: http://hotspotsystem.com/deskpro/kb_article.php?ref=5216-YGHX-8291