<apex:page standardController="Employee__c" extensions="testPage" tabStyle="Employee__c">
    
    <apex:form id="frm1">
    
    <div>
      <input type="radio" name="acc" checked="checked" value="Existing" onclick="pbBlockRenderJS('1');">Existing </input>
    
    
    
      <input type="radio" name="acc" value="NEWAcc" onclick="pbBlockRenderJS('2');">New</input>
    
      </div>
      
      
      
      
      <apex:pageBlock id="pb1" title="Pg Block 1" >
      <apex:pageBlockButtons >
     <apex:commandButton value="save" action="{!saving}"/>
       </apex:pageBlockButtons>
      
       <apex:pageBlockSection columns="1">
       <apex:inputField value="{!pme.Name}"/>
       <apex:inputField value="{!pme.Address__c}"/>
          <apex:inputField value="{!pme.Phone__c}"/>
    
       </apex:pageBlockSection>
      </apex:pageBlock>
    <apex:outputPanel id="pnl1">
    
      <apex:pageBlock id="pb2" title="Pg Block 2" rendered="{!IF(mypbBlock == '2',true,false)}">
      <apex:pageBlockButtons >
     <apex:commandButton value="save" action="{!saving}"/>
      </apex:pageBlockButtons>
      
       <apex:pageBlockSection columns="1">
       <apex:inputField value="{!pme.Name}" />
    <apex:inputField value="{!pme.Job_Title__c}"/>
       <apex:inputField value="{!pme.Salary__c}"/>
       
       </apex:pageBlockSection>
      </apex:pageBlock>
    
      </apex:outputPanel>
    
      <apex:actionFunction action="{!pbBlockRender}" name="pbBlockRenderJS" rerender="pb2">
    
            <apex:param name="firstParam" assignTo="{!mypbBlock}" value="" />
    
        </apex:actionFunction>
    
    </apex:form>
    
    </apex:page>

class

    public with sharing class testPage {
    public string Existing{get; set;}
    public string NEWAcc{get; set;}
    
        public Employee__c emp{get;set;}
        public Employee__c pme{get;set;}
        public Employee__c emplist{get;set;}
        public String var {get;set;}
            public boolean editSection {get;set;}
    
            public testPage(ApexPages.StandardController controller) {
                Employee__c emp=new Employee__c();
                pme=(Employee__c )controller.getRecord();
                editSection =true;
                var= ApexPages.currentPage().getParameters().get('Id');
                 system.debug('erecrrrtrtrt');
    
                    system.debug('erecrrrtrtrt'+NEWAcc);
                if(var!=null){
                emplist=[select Name,Address__c,Phone__c,Job_Title__c,Salary__c  From Employee__c where id=:var];
                system.debug('id'+var);
                }
                }
            public PageReference saving(){
                            system.debug('erecrrrtrtrt'+Existing);
    
                    system.debug('erecrrrtrtrt'+NEWAcc);
                upsert pme;
                PageReference p=new PageReference ('/'+pme.id);
                p.setRedirect(true);
                return p;
                }
        public PageReference pbBlockRender() {
        return null;
        }
        public PageReference EdittheSection (){
    editSection = true;
    return null;
    
            }
    
        public String mypbBlock { get; set; }
    }

I have two radio buttons,by default Existing is selected so Pg Block 1 is displayed works fine but am not able to display Pg Block 2 when New radio is selected,please help me how to solve it..Iam a fresher.