DomDocument nesne kullanarak xml dosyasını Midified?

Bir file.xml var

 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <Document xmlns="urn:iso:std">
 <content>
 <GrpHdr>
   <MsgId>xxxxxx </MsgId>
   <CreDtTm>2010-07-05T16:48:00</CreDtTm>
   <BtchBookg>false</BtchBookg>
   <NbOfTxs>2</NbOfTxs>
   <CtrlSum>56653</CtrlSum>
   <Grpg>Old</Grpg>
   <InitgPty>
   <Nm> Name </Nm>
   </InitgPty>
 </GrpHdr>
 </content>
 <Document>

Nasıl xml verileri modifiye edebilir ve (örneğin) geri file.xml kaydetmek

 <NbOfTxs>1</NbOfTxs>
 <CtrlSum>1223</CtrlSum>
 <Grpg>NEW</Grpg>