Change class="menu"

Where can I change the line <ul id=”menu-mainnavigation” class=”menu”> in <ul id=”menu-mainnavigation” class=”SUBmenu”> below the line <div class=”submenu-widget submenu-widget-subname”>?
This widget has the css style from the main menu what looks awful in the sidebar.

Thanks
Manuel

http://ww.wp.xz.cn/extend/plugins/gecka-submenu/