function ExecUpdate(endpointUrl, itemTerm) {
        var headers = {};
        headers["X-HTTP-Method"] = "MERGE";
        headers["If-Match"] = "*";

        headers["Accept"] = "application/json;odata=verbose";
        headers["X-RequestDigest"] = $("#__REQUESTDIGEST").val();
        var method = 'POST';
        var ajaxOptions =
        {
            url: url,
            type: method,
            contentType: "application/json;odata=verbose",
            headers: headers
        };
        if (typeof itemterm != 'undefined') {
            ajaxOptions.data = JSON.stringify(itemTerm);
        }
        return $.ajax(ajaxOptions);
    }

var listName = 'list name';  //set list name
    var itemId = 1; //set item id
    var webUrl = _spPageContextInfo.webAbsoluteUrl;
    var endpointUrl = webUrl + "/_api/web/lists/GetByTitle('" + listName + "')/items(" + itemId + ")";
    var fieldName = 'field name';  // field name
    var itemTerm = {
        '__metadata': { 'type': 'SP.Data.ContactsListItem' },
        fieldName: {
            '__metadata': { 'type': 'SP.Taxonomy.TaxonomyFieldValue' },
            'Label': 'set term label',  //set term label
            'TermGuid': '73ggdtes-8weg-6dsg-f982-9djksh6dvd34',  //set term guid
            'WssId': -1
        }
    };
    ExecUpdate(endpointUrl, itemTerm)