Go to Appearance -> Theme Options/ customise -> Design Options Tab -> Custom CSS paste the following CSS code and Click on save all changes button:
#access {
text-align: center;
}
#access li {
display: inline-block;
float: none;
margin-right: -4px;
}
#access a {
display: block;
float: none;
}
#access ul li ul {
text-align: left;
}
#access ul li ul li {
display: block;
margin-right: 0;
}