To do this you have to add:

using HP.LFT.SDK.StdWin;

and then in your code, put:

var TabRow = browser.Describe(new UiObjectDescription
{
AccessibleName = string.Empty,
WindowClassRegExp = @''DirectUIHWND''
}).Describe(new TabControlDescription
{
AccessibleName = @''Tab Row'',
AttachedText = string.Empty,
NativeClass = @''page tab list''
});

You can then use the index of the tab you want to select, starting with tab 0 like this:

TabRow.Select(0); //selects the first tab
TabRow.Select(1); //selects the second tab