Try this :
$.ajax({
url: 'webservices/LeadManagement.asmx/UpdateLeadmanage',
type: "POST",
dataType: 'json',
cache: false,
contentType: 'application/json; charset=utf-8',
data: "{'leadId':"+ leadid +",'enquiry':'"+enquiry+"','desc':'"+desc+"','category':'"+category+"'}",
success: function (result) {
alert(result.d);
},
error: function(err){ alert('error'); }
});