Within the Theme Editor, select Stylesheet from the theme files listed vertically to the right. The Style.css file should now be presented in the editor window and is ready for editing.

From the editor window, find the following code:

.narrowcolumn {
float: left;
padding: 0 0 20px 45px;
margin: 0px 0 0;
width: 450px;
}

and replace with:

.narrowcolumn {
float: right;
padding: 0 45px 20px 0px;
margin: 0px 0 0;
width: 450px;
}

Next, find:

#sidebar
{
padding: 20px 0 10px 0;
margin-left: 545px;
width: 190px;
}

and replace with:

#sidebar
{
padding: 20px 0 10px 0;
margin-left: 20px;
width: 190px;
}