Apex Controller Class

gistfile1.txt

/**
 * CustomSettingController
 * @description controller class for updating custom settings...
 * @date 8/2/2012
 */
public without sharing class CustomSettingController{

  public CustomSetting__c cs {get;set;}

  public CustomSettingController(){
    cs = CustomSetting__c.getInstance();
  }

  public PageReference update(){
    try{
      update cs;
      ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.CONFIRM,Label.CS_Update_Success));
    }catch(DMLException e){
      ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,Label.CS_Update_Error));
      System.debug(e);
    }
    return null;
  }
}

VF Page

<apex:page controller="CustomSettingController">
  <apex:pageMessages id="msgs" />
  <apex:form id="theform">
    <apex:inputField value="{!cs.Field1__c}" />
    <apex:inputField value="{!cs.Field2__c}" />
    <apex:commandButton value="{!$Label.CSTab_Save}" action="{!update}" rerender="msgs,theform" />  
  </apex:form>
</apex:page>