При клике на другой элемент данные старого удаляются и записываются новые.

function showCompanies(companies) { 
	for( var i = 0; i < companies.list.length; i++ ) { 
		var li = companiesList.appendChild(document.createElement('li')); 
		li.innerHTML = companies.list[i].name; 
 
		//show partners 
		//track 'li' number which has been clicked and show it partners 
		li.onclick = function(a) { 
			var a = a || window.event, 
			a = a.target || a.srcElement, 
			b = a.parentNode; 
			if (a.nodeName.toLowerCase() == "li" && b.nodeName.toLowerCase() == "ul") { 
				for (var current = 0, a = a.previousSibling; a && a != b;){ 
					a.nodeName.toLowerCase() == "li" && current++, a = a.previousSibling; 
				} 
			} 
			for (var j = 0; j < companies.list[current].partners.length; j++) { 
				document.getElementById('companyPartners').style.display = 'block'; 
				var partnersBlock = document.getElementById('partners'); 
				var p = partnersBlock.appendChild(document.createElement('p')); 
				p.innerHTML = companies.list[current].partners[j].name; 
			} 
		} 
 
	}; 
}