how to edit source code on wordpress

you are not alone –
http://wordpress.org/support/topic/menu-titletitle-problem-with-wp3-1

in this case it seems to have been a conflict with a seo plugin:
http://wordpress.org/support/topic/menu-titletitle-problem-with-wp3-2?replies=3

or – totally untested – try to edit header.php;
find:

<?php wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary', 'link_before' => '<span class="item-left"></span><span class="icon">' , 'link_after' => '</span><span class="item-right"></span>' ) ); ?>

change to:

<?php wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary', 'link_before' => "<span class='item-left'></span><span class='icon'>" , 'link_after' => "</span><span class='item-right'></span>" ) ); ?>

(make a backup copy before editing)

to answer your actual question:
http://codex.wordpress.org/Editing_Files