1. Visit http://wordpress.org/download/release-archive/ and download the version of WordPress you’d like to downgrade to.  When it’s done, unzip all the files.

2. FTP into your server and delete the wp-admin and wp-includes folders.  I would suggest FileZilla for a free FTP program.

3. Upload ALL the files that you just unzipped from Step 1 EXCEPT for the wp-content folder.  (Yes, overwrite everything)

4. Once all the files are uploaded, refresh your Dashboard and you will be asked to update your Database.  Click on the button (continue/yes/or whatever it says) and then you should be good to go.