Updating Salesforce Records Using JavaScript

Create a Opportunity Custom Button and select Behavior as Execute javascript and paste the below code,

{!REQUIRESCRIPT("/soap/ajax/35.0/connection.js")}

var opp = new sforce.SObject("Opportunity");
opp.Id = "{!Opportunity.Id}";
opp.StageName = "Closed Won";

//update the opp
result = sforce.connection.update([opp]);

if (result[0].getBoolean("success")) {
    log("opp.with id " + result[0].id + " updated");
} else {
    log("failed to update opp." + result[0]);
}

When user clicks that button it will make that opportunity as Won.