You could use code like the following, but, if you're updating whole paragraphs, you really should do so by changing the paragraph Style; even for individual characters, if they're not in the same font as the paragraph Style, you should redefine them using a character Style.
Code:
Sub UpdateDocuments()
Application.ScreenUpdating = False
Dim strFolder As String, strFile As String, wdDoc As Document
strFolder = GetFolder
If strFolder = "" Then Exit Sub
strFile = Dir(strFolder & "\*.doc", vbNormal)
While strFile <> ""
Set wdDoc = Documents.Open(FileName:=strFolder & "\" & strFile, AddToRecentFiles:=False, Visible:=False)
With wdDoc.Range.Find
.ClearFormatting
.Text = "[A-Za-z0-9]"
.Font.Name = "Times New Roman"
With .Replacement
.ClearFormatting
.Text = "^&"
.Font.Name = "Tahoma"
End With
.Forward = True
.Wrap = wdFindContinue
.Format = True
.MatchWildcards = True
.Execute Replace:=wdReplaceAll
End With
wdDoc.Close SaveChanges:=True
strFile = Dir()
Wend
Set wdDoc = Nothing
Application.ScreenUpdating = True
End Sub
Function GetFolder() As String
Dim oFolder As Object
GetFolder = ""
Set oFolder = CreateObject("Shell.Application").BrowseForFolder(0, "Choose a folder", 0)
If (Not oFolder Is Nothing) Then GetFolder = oFolder.Items.Item.Path
Set oFolder = Nothing
End Function
The above code will change all letters and numbers in all documents in the selected folder from Times New Roman to Tahoma. Since you haven't indicated anything substantial about what characters you're replacing, I can't provide much more help than that for the Find string.
PS: Don't store the file containing the macro in the same folder as the files to be processed.