Follwing code will open pwd protected file if the pwd is right

Dim wk As Workbook
Set wk = Application.Workbooks.Open("C:\test.xls", , , , "testpwd", True)
'True is required for editing the file

wk.Password = "newpwd"
wk.Save
wk.Close

For unprotected files use this and tehn set pwd
Dim wk As Workbook
Set wk = Application.Workbooks.Open("C:\test.xls", True)


wk.Password = "newpwd"
wk.Save
wk.Close