i have applied the following command in the config mode : service password-encryption
how can i remove it ? so that the passwords are no longer encrypted ?
Andriy is correct. Since these are VTY passwords they must be removed then re-added. You could also reverse the password since these passwords have weak encryptions.
Collin has given exactly the right answer. But it may not seem that way when you apply the no service password-encryption command. Be aware that this command turns off password encryption. But that passwords that have been encrypted and stored in the config as encrypted will remain encrypted even when you turn off the service. The only way to get those passwords into clear text is to configure the passwords again after the service for encryption is turned off.
Once encrypted passwords will stay so until you cancel encryption with no keyword AND enter them from command line again.