update users set password= md5('MyPassword') where password="myPassword";
It is not safe to use MD5 as a hash. MD5 is deprecated
Safe hash algorithms are PBKDF2, bcrypt, scrypt etc. And additionally all hash algorithms have to be used with salt.
@Gerald Schneider posted a very good link for this topic: https://security.stackexchange.com/questions/211/how-to-securely-hash-passwords
The problem here is, that none of these save hash algorithm are implemented in mysql. There is an authentication mode for PBKDF2 in ProFTPD in the mod sql module. But there is no way OOTB you can generate a PBKDF2 hashed password, or a password, that is hashed with another safe algorithm, in a mysql database.
A possible solution would be to create an HTML page with PHP. PHP has functions, implemented by default, for generating safe hashed passwords.
I asked in the ProFTPD forum, if anyone there knows another, maybe better answer, to the problem:
Yes, I know the original question was, how could one hash all the passwords in my database at once with one command. But I think, first, I should look for a safe hash algorithm.