var webUrl = _spPageContextInfo.webServerRelativeUrl;
 var listTitle = "mylist";
 var taxFieldName = "myColumn";
 var termLabel ="termm label";
 var termId = "a92b78b5-3505-45d5-abdd-b28e0b5385c5";

 var context = new SP.ClientContext(webUrl);
 var list = context.get_web().get_lists().getByTitle(listTitle);
 var item = list.getItemById(301);

 //In case of multi-valued Managed Metadata field, the format of value is:
 //-1;#<termLabel1>|<termId1>;-1;#<termLabel2>|<termId2>;...-1;#<termLabelN>|<termIdN>

 var termValue = "-1" + ";#" + termLabel + "|" + termId;

 item.set_item(taxFieldName, termValue);
 item.update();

 context.load(item);
 context.executeQueryAsync(
        function () {
            console.log('Item has been updated: ' + item.get_id());
        },
        function (sender, args) {
             console.log(args.get_message());
        });