Click 'Setup' and then 'Settings' in the main left menu. From there, click on 'Case Managers'.
Click on 'Invite Case Manager' and add the new user.
Once the new user (case manager) is added, move all cases from user (case manager) to be deleted/disabled to new user (case manager). For this go to Setup > Settings > Organization Tools > Transfers > Case Copy/Move
Once the cases are moved go to user (case manager) profile to be deleted/disabled and click on 'Disable' button.