The posts in Blogger are ordered in chronological order. So if you want to change the order of the posts, you will have to change the dates and/or time of the posts. In the Create or Edit posts mode, click the "Post and Comment Options" at the bottom left-hand corner and select the relevant date and time as shown in the screenshot.
If you want to reverse the chronology of the post using scripts, refer to Change order of Blogger posts
Note: Changing the date and time of the post will alter the url of the individual post page (permalink). So if you have any inbound links to that post, these links will be broken.
Update 1 April 2012: For the updated Blogger post editor, this option is now in the right sidebar of the post editor. You will have to click POST SETTINGS at the right sidebar, then click SCHEDULE (see screen shot below):
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Publishing a Blog with Blogger: Visual QuickProject Guide