It's one way hashing; you can't get back original form. It is not possible to decrypt a hash. What is the purpose though?
if you are just trying to compare this hash value with some other then you can do something like
IF HASHBYTES('md5', 'demo') = HASHBYTES('md5',@param)
PRINT 'Match!';
EDIT:
Hashing does not encrypt the original value at all. Hashing instead applies a one-way mathematical algorithm to the original value, resulting in a binary value.
It's mainly used for storing password. so, the password will be stored in hash format. When user provides credential (id/password); it doesn't decrypt the stored password; rather, user provided password will be hashed with the same algorithm and that hash value will be compared with the stored hash value (as shown a code example above).