Here is my apex class.
public class AccountOwnerUpdate{
    public Boolean isSelected{get;set;}
    public Account accounts{get;set;}
    public List<Account> selectedAccounts{get;set;}
   // public Boolean sendEmail{get;set;}
    public AccountOwnerUpdate(ApexPages.StandardSetController standardController)
    {   
        if(standardController.getSelected().size() == 0){
            isSelected = false;
            ApexPages.Message msg = new ApexPages.message(ApexPages.Severity.ERROR,'No Account Selected');
            ApexPages.addMessage(msg);
        }else{
            isSelected = true;
            selectedAccounts=  [SELECT id,name,ownerid from Account where id=:standardController.getSelected()];
           accounts = selectedAccounts[0]; 
           // accounts.ownerid = null; 
       }       
    }
    public PageReference updateAccount()
    {   
        for(Account a: selectedAccounts){
            a.ownerid = accounts.ownerid;
        }
        update selectedAccounts;
        return new Pagereference('/001/o');
    }

}

Here is my visual force page code. 

<apex:page standardController="Account" recordsetvar="Account" tabStyle="Account" extensions="AccountOwnerUpdate">
    <apex:pagemessages />
    <script>
        function myFunction() {      
            alert("Are you sure?");
        }
    </script>

    <apex:form rendered="{!isSelected}">
        <apex:sectionHeader subtitle="Change Owner" title="Account Edit" description="This screen allows you to transfer ownership of a Account to another user. When you transfer ownership of a Account, the new owner will own:
<br/>• all notes and attachments recorded for the current owner
<br/>• all open activities (tasks and events) owned by the current owner
<br/>
<br/>Note that completed activities and open activities owned by other users will not be transferred."/>
        <apex:pageblock mode="Edit" title="Account Edit">
            
    
           <apex:pageblockSection title="Select New Owner">
               <apex:inputField value="{!accounts.OwnerId}"/>
            </apex:pageblockSection>
           
            <apex:pageblockButtons >
                <apex:commandButton value="Save" action="{!updateAccount}" rendered="{!$User.Email == 'raj.sharma@theeclgroup.com'}"/>
                <apex:actionRegion >
                    <apex:commandButton value="Cancel" onclick="myFunction()" action="{!cancel}"/>  
               
                </apex:actionRegion>
            </apex:pageblockButtons>
        </apex:pageblock>
    </apex:form>
</apex:page>