To mortify, in older biblical English, means to put to death or make dead. In Paul’s teaching it describes the decisive destruction of the remnants of the old corrupt life in those united with Christ.
Paul uses the term of putting to death the deeds of the body and sinful members. This action is connected with the believer’s union with Christ and the new life that flows through the Spirit.
Modern versions replace “mortify” with “put to death.” The supplied explanation compares the process to removing diseased remnants after healthy circulation has been restored.