Use the following macro:

' Macro created 26/10/01 by Doug Robbins to update links in a document
'
Dim alink As Field, linktype As Range, linkfile As Range
Dim linklocation As Range, i As Integer, j As Integer, linkcode As Range
Dim Message, Title, Default, Newfile
Dim counter As Integer

counter = 0
For Each alink In ActiveDocument.Fields
   If alink.Type = wdFieldLink Then
       Set linkcode = alink.Code
       i = InStr(linkcode, Chr(34))
       Set linktype = alink.Code
       linktype.End = linktype.Start + i
       j = InStr(Mid(linkcode, i + 1), Chr(34))
       Set linklocation = alink.Code
       linklocation.Start = linklocation.Start + i + j - 1
       If counter = 0 Then
           Set linkfile = alink.Code
           linkfile.End = linkfile.Start + i + j - 1
           linkfile.Start = linkfile.Start + i
           Message = "Enter the modified path and filename following this
Format " & linkfile
           Title = "Update Link"
           Default = linkfile
           Newfile = InputBox(Message, Title, Default)
       End If
       linkcode.Text = linktype & Newfile & linklocation
       counter = counter + 1
   End If
Next alink