Use Workbooks.Open(filename) to open a file, and Workbook.SaveAs to save the file, specifying the format. The Visual Basic help has sample code for each method.
To open all files in a folder, try something like this:
(From
http://stackoverflow.com/q/11152870/190829)
Code:
Sub OpenFiles()
Dim MyFolder As String
Dim MyFile As String
MyFolder = "C:\Myfiles"
MyFile = Dir(MyFolder & "\*.wk1")
Do While MyFile <> ""
Workbooks.Open Filename:=MyFolder & "\" & MyFile
Loop
End Sub