You don't have to replace <br> by <br/> manually, if you need to close the node, just instruct the library to do so, for example this:

HtmlDocument doc = new HtmlDocument();
doc.LoadHtml("<br/>");
doc.Save(Console.Out);

will output this:

<br>

and this

HtmlDocument doc = new HtmlDocument();
doc.LoadHtml("<br/>");
doc.OptionWriteEmptyNodes = true;
doc.Save(Console.Out);

will output this:

<br />