Here in below example, there is a custom field Active__c in Contact object. On a button click want to update Active__c field status to true.
{!REQUIRESCRIPT("/soap/ajax/29.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/29.0/apex.js")}
if(confirm("Do you want to active this Contact?") == true)
{
var objC = new sforce.SObject("Contact");
objC.Id = '{!Contact.Id}';
objC.Active__c = true;
var result = sforce.connection.update([objC]);
window.location.reload();
}