Here is the procedure:
- 1) download the previous version at WordPress release archive
- 2) make a backup of (i) home folder (ii) wp-admin and (iii) wp-includes (*)
- 3) replace, using FTP, the current version of the mentioned folders and files with the version that you just downloaded (*)
- 4) long in WP and update the DB as requested
- 5) have fun!
(*) Note: do not touch the wp-content folder