XML : How to Edit XML node with same name using XDocument in C#

This is my XML

  <MilestoneCollection>      <Milestone>          <Description>Departure from Origin</Description>          <EventCode>DEP</EventCode>          <Sequence>1</Sequence>          <ActualDate></ActualDate>          <ConditionReference>LOC=&lt;FirstLeg.Origin&gt;</ConditionReference>          <ConditionType>RFP</ConditionType>          <EstimatedDate>2015-05-10T18:07:00</EstimatedDate>      </Milestone>      <Milestone>          <Description>Departure from Origin</Description>          <EventCode>DEP</EventCode>          <Sequence>1</Sequence>          <ActualDate></ActualDate>          <ConditionReference>LOC=&lt;FirstLeg.Origin&gt;</ConditionReference>          <ConditionType>RFP</ConditionType>          <EstimatedDate>2015-05-10T18:07:00</EstimatedDate>      </Milestone>      <Milestone>          <Description>Arrival at Destination</Description>          <EventCode>ARV</EventCode>          <Sequence>2</Sequence>          <ActualDate></ActualDate>          <ConditionReference>LOC=&lt;LastLeg.Destination&gt;</ConditionReference>          <ConditionType>RFP</ConditionType>          <EstimatedDate>2015-05-11T14:02:00</EstimatedDate>      </Milestone>      <Milestone>          <Description>Arrival at Destination</Description>          <EventCode>ARV</EventCode>          <Sequence>2</Sequence>          <ActualDate></ActualDate>          <ConditionReference>LOC=&lt;LastLeg.Destination&gt;</ConditionReference>          <ConditionType>RFP</ConditionType>          <EstimatedDate></EstimatedDate>      </Milestone>  </MilestoneCollection>    

This is my current code. I need to Edit the 1st, 2nd, or 3rd Milestone. I don't know how exactly the syntax for that. How do i edit element Milestone[0] something like this.

  XDocument doc = XDocument.Load(sample.xml);  var xmldocu = doc.Descendants("MilestoneCollection");