I have a website, it shows single webpages at a time?

[ABC]

Sweet Heart your website is using j__doPostBack function to call a url, which is causing you to opening url in a another tab.

You code is.
[href="j__doPostBack('Header1$GbmImage','')"><span>GBMhomebroker</span></a></h1>
</div>
<div style="margin-top: -1px; float: right; padding-right: 18px;">
<ul id="nav">
<li>
<a id="Header1_lblBlog" class="MenuSuperItem2" CommandArgument="Blog" href="https://www.gbmhomebroker.com/blog/" target="_blank">Blog</a></li>
<li>
<a id="Header1_lblHerramientas" class="MenuSuperItem2" href="j__doPostBack('Header1$lblHerramientas','')">¿Por qué HB?</a></li>
<li>
<a id="Header1_lblAnalisis" class="MenuSuperItem2" href="j__doPostBack('Header1$lblAnalisis','')">Análisis</a></li>
<li>
<a id="Header1_lblTrOportunidades" class="MenuSuperItem2" href="j__doPostBack('Header1$lblTrOportunidades','')">Análisis Técnico</a></li>
<li>
<a id="Header1_lblReportes" class="MenuSuperItem2" href="j__doPostBack('Header1$lblReportes','')">Reportes</a></li>
<li>
<a id="Header1_lblPRO" class="MenuSuperItem2" href="j__doPostBack('Header1$lblPRO','')">PRO</a></li>
<li>
<a id="Header1_lblCostos" class="MenuSuperItem2" href="j__doPostBack('Header1$lblCostos','')">Costos</a></li>
<li>
<a id="Header1_lblQuien" class="MenuSuperItem2" href="j__doPostBack('Header1$lblQuien','')" style="padding-right: 0px !important;">Nosotros</a></li>
]

Whereas your code should be look like this to open in a new tab. But dont worry you can adjust it using your c panel.

[<li id="hometab"><a href="/pages/index.aspx"><span></span>Home</a> </li>
<li id="aboutUs"><a href="/pages/aboutus/overview.aspx" class="drop"><span></span>About Us</a>

<!--2nd level-->
<ul class="sub_menu">
<li>
<h3><a href="/pages/aboutus/from-ceos-desk.aspx">From CEO's Desk</a></h3>
</li>
<li>
<h3> <a href="/pages/aboutus/fact-sheet.aspx">Fact Sheet</a> </h3>
</li>
<li>
<h3><a href="/pages/aboutus/leadership.aspx">Leadership</a></h3>
</li>
<li>
<h3><a href="/Pages/aboutus/partnerships.aspx">Partnerships</a></h3>
</li>
<li>
<h3><a href="/pages/aboutus/testimonials.aspx">Testimonials</a> </h3>
</li>
<li>
<h3><a href="/pages/aboutus/infrastructure.aspx">Infrastructure</a></h3>
</li>
<li>
<h3> <a href="/pages/methodologies/methodology.aspx">Methodologies</a> </h3>
</li>
]
Hope this help.