If this is a one-off task, use Seph's answer. If it needs to be repeated, use VBA, like this:
Dim iRow As Long, LastRow As Long
Dim sh As Worksheet
Set sh = ActiveSheet ' <-- Maybe change this to refer to a specific sheet
Application.FindFormat.Clear ' Just in case Find was last used with a Format
LastRow = sh.Cells.Find(What:="*", After:=sh.Cells(1, 1), _
SearchDirection:=xlPrevious, SearchOrder:=xlByRows, SearchFormat:=False).Row
For iRow = LastRow To 1 Step -1
' Delete even numberede rows (to delete odd numbered
' rows, instead use If iRow Mod 2 = 1 Then).
If iRow Mod 2 = 0 Then
Make sure you back up before running any macro on your sheet. Macros don't use Undo!