In the general case, you can't, as BrowserUK says, md5 is digest, not a cypher so you can't decrypt.
If the list of plaintexts is small and known, you could construct a lookup table that will allow you to convert from one to the other, but that is limited by the time it takes to pre-compute hashes, and the disc space to store them all. Likewise if you can guess plaintext, then you can check your guesses.
There are services online that have access to big lookup tables to decrypted hashed passwords, though they are only suitable for short common passwords, and even then don't work very well. For example I tested with the md5 hash for "password" and "secret". md5decrypter.co.uk successfully decrypted "secret", while md5crack.com was not able to crack either hash.
I should also remind you that depending on the circumstances, cracking passwords in this way may be illegal, unethical, or against the rules of your employer. You should tread with caution, and seek advice before attempting to reverse any md5 hashed data or password that you did not originate yourself.