Updating Pick list values via metadata API help required

[Apex Learner]

hi i have woring on METADATA Api and soap ui

and i have successfully create soap envelope to add new picklist value in picklist field

via

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:met="http://soap.sforce.com/2006/04/metadata">
   <soapenv:Header>
      <met:CallOptions>
         <met:client>---</met:client>
      </met:CallOptions>
      <met:SessionHeader>
         <met:sessionId>---</met:sessionId>
      </met:SessionHeader>
   </soapenv:Header>
   <soapenv:Body>
      <update xmlns="http://soap.sforce.com/2006/04/metadata">
     <!--Zero or more repetitions:-->
      <UpdateMetadata xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="customfield">
		    <currentName>lead.number__c</currentName>
		                
		      <metadata xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CustomField">
		      <!--Optional:-->
		       <fullName>lead.number__c</fullName>
		       <label>number</label>
		    		<picklist>
		              <picklistValues>
		         			<fullName>new</fullName>
		              </picklistValues>
		         		 <sorted>false</sorted>
		  		</picklist>
		         <type>Picklist</type>
		    </metadata>
         </UpdateMetadata>
       </update>
   </soapenv:Body>
</soapenv:Envelope>

 Now issues I am facing , is

Is there any way to update / edit any particular picklist value ???

e.g , Object : Lead , field : number__c, type : picklist , picklist values : new , old 

any way i can edit "new " to "new_value" via meta data API .

Thanks to soap and metadata api developers in advance