Normally the advice here would be to make a child theme, but as you seem to have your own custom theme I’m going to assume you can just edit your code directly.
In the file style.css find this piece of code at line 176:
.menu ul
{
margin: 0;
padding: 0;
list-style-type: none;
list-style-image: none;
float: right;
margin-top: 31px;
}
Replace it with this:
.menu ul
{
margin: 0;
padding: 0;
list-style-type: none;
list-style-image: none;
float: left;
margin-top: 120px;
margin-bottom: 20px;
}
Find the #header selector in line 132 and delete this line:
border-bottom: 5px solid #990000;
Your original theme author seems to have made some slightly odd choices that make the theme tricky to modify, but that should work for now.