You do that using MoveFocus as shown in this MSDN article which explains everything about focus: Focus Overview.
Here is some sample code to get to the next focused element (got it from that article, slightly modified).
// MoveFocus takes a TraversalRequest as its argument.
TraversalRequest request = new TraversalRequest(FocusNavigationDirection.Next);
// Gets the element with keyboard focus.
UIElement elementWithFocus = Keyboard.FocusedElement as UIElement;
// Change keyboard focus.
if (elementWithFocus != null)
{
elementWithFocus.MoveFocus(request);
}
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