You can use my toolkit as mentioned above to change passwords, but its impossible to "decode" a hash. Its a one way encryption, so changing it is the only way to go.
Admin ToolKit v2.1a - An Admins most helpful tool for user management. Now Supports Mass User Deletion!
Change User's: names, passwords, emails, active status and avatar/pm permissions.
Ban/Unban Users, change Post and Resync Counts, and promote/demote users to admin.
Completely independent from your phpbb user account settings. No installation required, just upload one file.
User Upload ToolKit Beta - A quick and easy, 30 second-install, attachment mod. Now Supports Dynamic Thumbnails!