Complex sentence: Unfortunately, the thousands of visitors every year walking to the top of it had shifted the sand and the dune was "demoted" as the highest point, which is now the "White Dune" situated on the neighbouring island of Spiekeroog.
Split into simpler sentences: Unfortunately, the thousands of visitors every year walking to the top of it have shifted the sand and the dune was "demoted" as the highest point. The highest point is now the "White Dune" on the neighbouring island of Spiekeroog.