The other answer is incomplete. UpdateQueryAsync was missing which does the actual write opration on the server. Below is complete piece of code.

function UpdateItem() {
clientContext = new SP.ClientContext.get_current();
oList = clientContext.get_web().get_lists().getByTitle(‘ListName’);

this.oListItem =oList.getItemById(1);

oListItem.set_item(‘Title’, ‘My Updated New Title’);

oListItem.update();

clientContext.load(oListItem);

clientContext.executeQueryAsync(
Function.createDelegate(this, this.onQueryInsertSucceeded),
Function.createDelegate(this, this.onQueryInsertFailed));
}

function onQueryInsertSucceeded() {
alert(‘Item Updated: ‘);
}

function onQueryInsertFailed() {
alert(“Item Update Failed”);
}

function updatelistitem() {
 var clientContext = new SP.ClientContext(https://****.sharepoint.com/sites/demo");
 this.oListItem = oList.getItemById(3);
 var oList = clientContext.get_web().get_lists().getByTitle('Announcements');
    oListItem.set_item('Title', 'My Updated Title');
    oListItem.update();
}