Well 99% of all site systems hash the passwords for safety reasons. If someone hacks your database he wont be able to read the passwords.
The fact that these are one way encryptions means you can not decode them or decrypt them. The only way to receive the original password is by bruteforcing which tries every combination possible (it will take months to do this...)
The hashing depends on your CMS though most systems only hash in md5 or sha1.
If you've figured that out you can go to:
MD5 Encrypt - MD5Decrypter.co.uk, MD5 Decryption, Free MD5 Decryptor, MD5 Cracker, MD5 Hash and hash a password. Copy the encrypted password and paste it in the database.
If you have a custom hashing you can let them create a new account + password and then retrieve the password from the database and update the users original account (assuming you want to reset it.).
Cheers!