ActiveWorkbook.ActiveSheet.Name = "newname"

Sub WorkSheetNameChange()

Worksheets("Sheet2").Name = "New Name"
End Sub

Or you may refer to the sheet  by its internal number. But perhaps you
should first read up more about the object model in the VBE Help file.
Start by searching for worksheet.

Or you can use

Worksheets(x).Name = "new name"

where x is the position of the worksheet.